Armed servicemen stand near Russian Army vehicles outside a Ukrainian border-guard post in the Crimean town of Balaclava.
The Group of Seven (G7) leading industrialized countries say they are suspending participation in the planning for an international summit in Russia.
The decision comes as tensions are increasing in Crimea after Russia's parliament on March 1 approved the deployment of troops to Ukraine and Kyiv a day later ordered the mobilization of reservists.
The White House issued a joint statement on March 2 on behalf of the G7 leaders, the president of the European Council, and the president of the European Commission.
In the statement, the leaders condemned Russia’s "clear violation of the sovereignty and territorial integrity of Ukraine," adding that Moscow's actions violate the "principles and values" on which the G7 and G8 operate.
The G7 includes Britain, Canada, France, Germany, Italy, Japan, and the United States. The countries also participate in the G8, which includes Russia. The country's Black Sea port of Sochi is scheduled to host the G8 summit in June.
The leaders also expressed support for Ukraine’s efforts to negotiate a deal with the International Monetary Fund to stave off an economic crisis.
LIVE BLOG: Ukraine Crisis
Earlier, Ukrainian Prime Minister Arseniy Yatsenyuk said "we are on the brink of disaster" and urged Russian President Vladimir Putin to pull back his military from the country.
Speaking to foreign reporters in Kyiv, Yatsenyuk said: "This is the red alert. This is not a threat -- this is an actual declaration of war on my country."
WATCH: Armed men in unmarked uniforms patrol in Simferopol.
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Soldiers In Unmarked Uniforms Patrol In Crimea
The head of Ukraine's National Security and Defense Council, Andriy Parubiy, ordered the Defense Ministry to "call on all those that armed forces need at the moment across Ukraine," adding that the mobilization was to "ensure the security and territorial integrity of Ukraine."
Parubiy also said an order had also been given to the Foreign Ministry to seek U.S. and British help in guaranteeing the security of Ukraine.
The 1994 Budapest Memorandum signed by Russia, the United States, and Britain engaged all three powers to respect the independence, sovereignty, and territorial integrity of Ukraine.
The situation has been tense in Crimea, with Russian forces consolidating their hold on the Black Sea peninsula and telling Ukrainian forces there to give up their weapons.
Ukraine's new government has launched a treason case against the head of the navy after he surrendered his headquarters in the Crimean port of Sevastopol.
The government in Kyiv only made Denis Berezovsky the head of the navy on March 1. http://english.share.voanews.eu/flashembed.aspx?t=phg&id=1862452&w=640&h=506&skin=embeded
Berezovsky was shown on Russian television on March 2 swearing allegiance to Serhiy Aksyonov, the pro-Russian politician recently elected by Crimea's regional parliament as prime minister.
Berezovsky was dismissed shortly afterward by Kyiv and a treason case launched against him. Aksyonov announced he had given orders to Ukrainian naval forces on the peninsula to disregard any orders from the "self-proclaimed" authorities in Kyiv.
International Concern
Meanwhile, U.S. Secretary of State John Kerry said Russia's "incredible act of aggression" in Ukraine amounts to "a stunning willful choice" to invade that country on a "trumped-up pretext."
Kerry, appearing on the CBS network's program "Face the Nation," also threatened "very serious repercussions" from the United States and other countries including sanctions to isolate Russia economically.
Ukrainian navy chief Denis Berezovsky swore allegiance to the pro-Russian regional leaders in Crimea.
Kerry is to travel to Kyiv on March 4 to lend support to Ukraine’s new interim leaders.
The State Department said he will "reaffirm the United States’ strong support for Ukrainian sovereignty, independence, territorial integrity, and the right of the Ukrainian people to determine their own future, without outside interference or provocation.”
U.S. Treasury Secretary Jack Lew told a conference in Washington that the United States "is prepared to work with its bilateral and multilateral partners to provide as much support as Ukraine needs."
In Brussels, NATO chief Anders Fogh Rasmussen called on Russia to withdraw its forces from Ukraine and refrain from interferring in other parts of that country.
NATO's secretary-general was speaking to reporters after chairing an emergency meeting of ambassadors from the 28 NATO member states, known as the North Atlantic Council.
After almost eight hours of talks, the alliance issued a statement urging “both parties to immediately seek a peaceful resolution through dialogue, through the dispatch of international observers under the auspices of the United Nations Security Council or the OSCE."
The meeting of the North Atlantic Council, the alliance’s main political decision-making body, was at the request of Poland.
Warsaw has said it feels threatened by any potential Russian military intervention in neighboring Ukraine.
Also on March 2, Germany said President Putin agreed to a proposal to "immediately establish a mission of enquiry as well as a contact group, possibly under the direction of the OSCE, to open a political dialogue."
The German government statement said Chancellor Angela Merkel raised the idea in a phone conversation.
U.S. Assistant Secretary of State for Europe Victoria Nuland is to travel to Vienna on March 3 for talks with the Organization for Security and Cooperation in Europe.
A U.S. official was quoted as saying she was hoping to look at ways to get international monitors into flashpoint areas and possibly "a broader mission to replace Russian forces if the Russians can be persuaded to pull back."
The European Union said EU foreign ministers will hold urgent talks over Ukraine on March 3.
