President Volodymyr Zelenskiy thanked Poland for rallying Western support for Ukraine and said that Russia will not defeat Europe as long as Ukraine and Poland are working closely together.
Zelenskiy, who visited Warsaw on April 5, and Polish Prime Minister Mateusz Morawiecki signed a letter of intent to cooperate on the supply of defense equipment and ensure constant military support, the press service of the Ukrainian president said.
"Today we are signing a document on the supply of Polish Rosomak armored personnel carriers, Rak self-propelled mortars, air-defense systems, in particular the very effective Peruns, MiGs, and other weapons that we badly need," Zelenskiy said at a news conference after talks with Morawiecki.
WATCH: Ukrainian assault brigade sappers risk everything to clear the front line of mines laid by Russian forces -- and lay their own. They often work a few hundred meters from Russian positions, leaving them vulnerable to attack. As one sapper told Current Time correspondent Oleksiy Prodayvoda about a recent mission: "I thought I wasn't coming back."
Your browser doesn’t support HTML5
Praying In The 'Gray Zone': Ukraine's Frontline Sappers
The two leaders also signed an agreement on cooperation in the reconstruction of Ukraine's infrastructure and discussed the creation of joint enterprises for the production of weapons and ammunition.
"Despite everything, the defense-industrial complex of our countries will work effectively," Zelenskiy said, describing the agreements signed during his visit as "very substantive."
Later, in a speech at Warsaw's Royal Castle, he thanked Poland for the significant military aid, expressing gratitude to each and every Polish person for this "brotherhood" and saying, "There is no force left that can overcome Ukrainian-Polish friendship."
Live Briefing: Russia's Invasion Of Ukraine
RFE/RL's Ukraine Live Briefing gives you the latest developments on Russia's invasion, Western military aid, the plight of civilians, and territorial control maps. For all of RFE/RL's coverage of the war, click here.
Zelenskiy called the Polish-Ukrainian alliance a cornerstone of freedom for all Eastern European countries battling Russian imperialism.
"If we are free together with you, it is the guarantee that freedom will be strong among all our neighboring countries, the neighbors of the European Union -- Romania, Slovakia, Lithuania, and other Baltic countries. All of them are stronger when we are free," Zelenskiy said.
"If we are free, that serves as the guarantee that freedom will prevail in Moldova, too, that it [will] not leave Georgia, and will definitely come to Belarus," he said.
Zelenskiy said earlier that Ukraine regards Poland as one of its closest Western allies and a critical partner in its reconstruction after the war with Russia ends.
Polish President Andrzej Duda told the news conference that Poland will keep helping Ukraine fight off Russia's aggression and pledged to give Ukraine more MiG-29 fighter jets.
"Four MiG-29s that remained in storage have been handed over to the Ukrainian armed forces in recent months. Four MiGs are now being given, so eight in total," Duda said. "We are ready...to give six more that are currently being prepared. We assume they could be transferred soon."
Duda also said that Warsaw is working toward obtaining additional security guarantees for Ukraine at a NATO summit to be held in the Lithuanian capital, Vilnius, on July 11-12.
"Today, we are trying to obtain for Ukraine at the next NATO summit in Vilnius... additional security guarantees that will strengthen the military potential of Ukraine," Duda said.
As Ukrainian forces continue to put up a stern resistance in the east, where Russian troops have been attempting for months to capture Bakhmut and several other key cities in the Donetsk region, Zelenskiy told the news conference in Warsaw that the situation continues to be difficult and that "corresponding decisions" would be made if Ukrainian forces there risk being encircled.
Zelenskiy said that protecting the lives of Ukrainian soldiers was the most important thing to him.
Poland has been instrumental in mobilizing Western military and political support for its eastern neighbor since the start of Russia's unprovoked invasion more than one year ago.
The country has also been host to some 1 million Ukrainian refugees, and Zelenskiy, accompanied by Duda, is due to meet at Warsaw's Royal Castle with some of the Ukrainians who have found shelter in Poland.
Meanwhile, four civilians were killed and 11 others were wounded in the Donetsk region by Russian shelling overnight, the region's governor, Pavlo Kirylenko, said on April 5.
Three people were wounded by Russian bombardments in the southern region of Kherson, the regional administration said, and one person was wounded in the Sumy region, local officials reported.
Ukrainian defenders repelled more than 60 Russian attacks over the past 24 hours, the General Staff of the Armed Forces of Ukraine reported in its daily bulletin on April 5, adding that most attacks continued to focus on Bakhmut, Avdiyivka, and Maryinka.
Russian forces also launched an unsuccessful assault on Bohdanivka in the Donetsk region, the military said. A total of 17 Iranian-made drones were launched by Moscow's forces overnight, the military reported, without giving details.
Trenches Of New York: Troops Hold The Line Near Ukrainian Village
On the diplomatic front, French President Emmanuel Macron, who arrived on a visit to China on April 5, told journalists in Beijing that anyone helping "aggressor" Russia in the Ukraine conflict would become an "accomplice."
"We have decided since the beginning of the conflict to help the victim, and we have also made it very clear that anyone helping the aggressor would be an accomplice in breach of international law," he said.
French officials have said that Macron planned to urge Chinese President Xi Jinping during talks on April 5 to use Beijing's influence with Russian President Vladimir Putin to find a solution to the conflict.
Macron voiced hope that Beijing will “participate in initiatives that are useful to the Ukrainian people,” the officials have said.