The U.S. ambassador to the United Nations has strongly condemned Russia, saying there is nothing to justify its military presence in neighboring Ukraine.
At an emergency session of the U.N. Security Council Monday, Ambassador Samantha Power called Russia's intervention an "act of aggression," and not the humanitarian mission that Moscow is seeking to portray.
Power told the Council that Russia has every right to wish events had turned out differently. But she said it does not have the right to express that using military force.
Russian Ambassador Vitaly Churkin read a statement from Ukraine's ousted pro-Russian president Viktor Yanukovych in which he asked Moscow to intervene militarily. The statement said the intervention was needed to restore peace and stability to Ukraine.
The Security Council meeting came less than a week after Yanukovych fled Ukraine as anti-government protests gripped the country.
Earlier Monday, U.S. President Barack Obama warned of diplomatic and economic penalties if Moscow does not withdraw its forces. Mr. Obama said Russia is "on the wrong side of history" in Ukraine.
In tandem with the U.S. diplomatic push, the Pentagon said Monday it is suspending military-to-military contacts with Russia. Pentagon spokesman Rear Admiral John Kirby said the move is aimed at prodding Moscow to deescalate the Ukraine crisis, and said the suspension covers maneuvers, bilateral meetings, port visits and planning conferences.
Secretary of State John Kerry was scheduled to arrive in the Ukrainian capital Tuesday to demonstrate what U.S. officials say is a show of support for Ukrainian democracy.
Ambassador Power told the Security Council Russia is moving troops into Ukraine's Crimea Peninsula by ferry, after Russian soldiers on Monday seized a border post on the Ukrainian side of a waterway separating the two countries.
U.S. Vice President Joe Biden spoke Monday by telephone with Russian Prime Minister Dmitry Medvedev. The White House said Biden urged Russia to withdraw its forces from Ukraine, support the immediate deployment of international monitors and begin a "meaningful political dialogue" with the Ukrainian government.
European Union Council President Herman Van Rompuy said EU leaders will hold an extraordinary meeting on the Ukraine crisis Thursday in Brussels.
Russia's foreign ministry on Monday strongly criticized the Group of Eight major industrialized nations for suspending preparations on the G8 summit schedule to take place in the Russian city of Sochi in June. The ministry said the move damages not only the G8 countries, but "the whole international community."
Crimea is a Black Sea peninsula placed under Ukrainian control in 1954 by then-Soviet leader Nikita Khrushchev. It remained part of Ukraine when the Soviet Union collapsed in 1991. Crimea has a tiny border with Russia on its far eastern point. The Crimean port of Sevastapol is home to Russia's Black Sea fleet. Most of the people living in Crimea are ethnic Russians, but the region also is home to ethnic Muslim Tatars who generally show disdain for Russia.
Ukraine's troubles began in November when President Yanukovych backed out of a trade deal with the European Union in favor of closer ties and economic aid from Russia. The move triggered weeks of pro-Western demonstrations in Kyiv and elsewhere in Ukraine, and forced the pro-Russian Yanukovych to flee the capital in late February.