Zelenskiy Says Ukraine Has Suffered 'Serious Losses' After Russian Air Strikes Pound Dozens Of Targets

President Volodymyr Zelenskiy has told his nation that Ukraine suffered "serious losses" in the initial stages of Russia's long-feared attack early on February 24, with an adviser saying at least 40 people had been killed.

Zelenskiy also said he was breaking diplomatic ties with Moscow and he urged Russians "who had not lost their honor" to protest their government's actions.

Zelenskiy spoke in a televised address on February 24 shortly after Russian missiles had targeted air bases and military infrastructure in at least 25 cities after Russian President Vladimir Putin ordered a "special" operation to "demilitarize" its neighbor and rival.

The attack began in the early hours of February 24, with explosions and sirens being heard in the capital, Kyiv, and at its Boryspil airport, and witnesses reporting missile blasts in many other cities, including Kharkiv, Kramatorsk, Dnipro, Mariupol, Odesa, and Zaporizhzhya.

The attack prompted outrage and denunciations from the West and elsewhere as fears grew of a major war in the middle of Europe.

NATO ambassadors said the alliance had agreed to beef up its land, sea, and air forces on its eastern flank near Ukraine and Russia after Putin ordered the attack on Ukraine.

NATO ambassadors said in a statement after emergency talks on February 24 that "we have increased the readiness of our forces to respond to all contingencies."

Zelenskiy -- who spoke by phone with U.S. President Joe Biden and British Prime Minister Boris Johnson -- declared martial law, but he urged citizens to remain calm amid the Russian assault.

He said Moscow had launched a full-scale attack on Ukraine, with missile attacks targeting "our military infrastructure" and border guards in several cities.

WATCH: Russian missiles and air strikes targeted Ukrainian military positions and infrastructure across the country. Videos posted to social media showed explosions in several cities as Russia launched its attack of Ukraine.

Russian Missiles, Air Strikes Hit Ukrainian Targets

The Kremlin on February 24 repeated Putin's previous statement, saying that the goal of the invasion was the "demilitarization and de-Nazification of Ukraine."

Russian Armor 'Destroyed' On Outskirts Of Kharkiv

Earlier on February 24, Putin justified invading Ukraine by saying that Moscow would try to demilitarize and "de-Nazify" Ukraine, reiterating his baseless accusations made in a speech on February 21 that the Ukrainian government was a "neo-Nazi" regime that was seeking a nuclear weapon.

Explaining what the demilitarization of Ukraine means, Peskov said it was the "neutralization of its military potential, which recently was noticeably increased due to, among other things, the active efforts by foreign countries."

WATCH: Kyiv residents spoke to RFE/RL's Ukrainian Service as Russian missiles hit cities across the country. Some Ukrainians are hoping to flee to safety, but others plan to stay at home.

Traffic Jams And Lines For Cash As Kyiv Reacts To Russia's Attack On Ukraine

Answering a question about how deep Russia plans to get inside Ukraine, Peskov said: "I cannot give any information related to military, technological, and other elements of this operation. The only source for that data is our defense institution, our military."

"The terms of the operation will be defined by its efficiency, congruity, and will, of course, be defined by the supreme commander."

Zelenskiy, meanwhile, urged Ukrainians to step up to defend their country and promised to provide them with weapons if they did so.

Scenes From An Invasion: Russia Launches Long-Predicted Attack Against Ukraine

Putin "wants to destroy my country, our country," he said in an emotional address.

Ukrainian Foreign Minister Dmytro Kuleba urged other countries to impose tough sanctions on Russia immediately and to provide Ukraine with military and financial support.

"Right now, Putin is plunging Europe into its darkest time since 1939. Any government hoping to sit this out is naive. Don't repeat mistakes of the past," Kuleba said on Twitter.

SEE ALSO: How Long Could Ukraine Hold Out Against A New Russian Invasion?

Ukraine closed its airspace to civilian flights on February 24 while Europe's aviation regulator warned about hazards to flying in bordering regions.

Moldova, southwest of Ukraine, also closed its airspace, while Belarus to the north said civilian flights could no longer fly over part of its territory after Putin authorized the military operation.

CNN reported that Russian troops had entered Ukraine through the port city of Odesa, but it was not clear if they were involved in fighting. CNN also showed video purporting to show Russian troops entering Ukraine from Belarus, where they had been involved in military drills.

As of midday, an adviser to Zelenskiy said Ukraine was being hit by a second wave of missile strikes. No further details were immediately available.

Russia's Defense Ministry said it had struck military infrastructure at Ukraine's air bases and "suppressed" its air defenses but that it had not hit populated areas.

The Ukrainian Defense Ministry said that "heavy fighting" was under way in the Kharkiv area and in the regions close to the separatist-held areas in eastern Ukraine.

"The enemy suffered losses in troops and equipment. Our troops regained full control over the towns of Mariupol and Shchastya.... At least six planes, two helicopters, and tens of armored vehicles of the enemy were destroyed," it said, although the claims could not immediately be confirmed.

There was immediate and widespread condemnation from the West, with vows of new, tougher sanctions to be slapped on Moscow.

Biden called the action an "unprovoked and unjustified" attack on Ukraine and said the world would "hold Russia accountable."

"The prayers of the entire world are with the people of Ukraine tonight as they suffer an unprovoked and unjustified attack by Russian military forces," Biden said in a statement.

"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering. Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way. The world will hold Russia accountable," he added.

WATCH: Ukrainian authorities released video showing Russian tanks and military vehicles crossing a checkpoint from the occupied Crimean Peninsula on February 24:

Russian Tanks, Military Vehicles Roll Through Crimean Checkpoint

The White House said later that Biden had spoken with Zelenskiy and had offered his support and briefed the Ukrainian leader on the planned next steps against Russia by Washington and its allies.

Biden said he would address the nation on February 24.

British Prime Minister Johnson condemned the "horrific events in Ukraine" and said Putin "has chosen a path of bloodshed and destruction by launching this unprovoked attack."

"The U.K. and our allies will respond decisively," he wrote on Twtiter, adding that he -- like Biden -- had spoken by phone with Zelenskiy.

The EU echoed those remarks, with European Council President Charles Michel and European Commission President Ursula von der Leyen saying that "by its unprovoked and unjustified military actions, Russia is grossly violating international law and undermining European and global security and stability."

NATO Secretary-General Jens Stoltenberg condemned what he called Russia's "reckless" attack on Ukraine.

He said NATO allies will meet to address "Russia's renewed aggression." Ukraine is not a member of NATO but is aligned with the West.

"I strongly condemn Russia's reckless attack on Ukraine, which puts at risk countless civilian lives. This is a grave breach of international law and a serious threat to Euro-Atlantic security," Stoltenberg wrote on Twitter.

European Union leaders will discuss new, tougher sanctions on Russia at an emergency meeting later on February 24 in reaction to its "barbaric attack" on Ukraine, von der Leyen said.

"We will hold President Putin accountable for that," she said.

"With this package, we will target strategic sectors of the Russian economy by blocking their access to key technologies and markets," von der Leyen said in a statement to the media. "We will weaken Russia's economic base and its capacity to modernize."

EU foreign policy chief Josep Borrell said the bloc will slap the harshest package of sanctions ever implemented on Russia.

SEE ALSO: In Dark Remarks On Ukraine, Putin Telegraphed A Trajectory Toward War

"The European Union will respond in the strongest possible ways...(EU leaders) will adopt a stronger package, the harshest package of sanctions we have ever implemented," he told reporters in Brussels.

Hungary's prime minister, Viktor Orban, who has in the past pursued close ties with Putin, also criticized Russia's move against its smaller neighbor. "Together with our European Union and NATO allies, we condemn Russia's military action," he said in a video on Facebook.

Orban, whose anti-migrant policies have put him at odds with the EU, also said that the number of Ukrainian refugees approaching Hungary's borders was likely to grow. He added that Hungary was "prepared to care for them and will be able to meet this challenge quickly and effectively."

In a nationally televised speech early on February 24, Putin sought to justify the offensive operation by claiming that he had to stop Ukraine from acquiring nuclear weapons. Ukraine has not indicated it is seeking nuclear weapons.

"Circumstances require us to take decisive and immediate action," Putin said, and that Moscow-backed separatists in Ukraine's Donbas region had asked for assistance.

Putin also said the action was intended to protect civilians and that it came in response to threats coming from Ukraine -- claims Kyiv and the West have long dismissed.

Putin called on the Ukrainian military to lay down its arms. He claimed that Russia did not intend to occupy Ukrainian territory.

Russia had massed more than 150,000 combat-ready armed forces with heavy equipment on Ukraine's border, something NATO has described as the largest military buildup on the European continent since the end of the Cold War.

Putin had for weeks denied any plans to invade Ukraine.

Satellite Photos Show Bolstering Of Russian Forces And Supplies Around Ukraine Amid Fears Of Full-Scale Invasion

The Russian leader is seeking to prevent Ukraine from joining NATO -- something he has called an existential threat -- and bring the Western alliance's eastward expansion to a halt.

Ukraine has been aiming to join NATO ever since Russia seized its Crimean Peninsula in 2014 and backed separatists in its eastern provinces.

Zelenskiy made a last-ditch effort to avoid a war by trying to call Putin late on February 23. Putin did not take the call, Zelenskiy said.

With reporting by AP, AFP, Reuters, CNN, and the BBC