European and U.S. officials called Monday on Russia to withdraw its troops from Crimea, as Russian officials denied reports of plans to attack Ukrainian forces on the peninsula.
The European Union Foreign Affairs Council said it "strongly condemns" what it called "the clear violation" of Ukrainian sovereignty and territorial integrity by Russian armed forces, which it said had committed "acts of aggression."
EU Council President Herman Van Rompuy said EU leaders will hold an extraordinary meeting on the Ukraine crisis Thursday in Brussels.
Both U.S. and European officials were quoted Monday as saying punitive measures against Russia are being considered if it fails to take steps to defuse tensions.
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.
Russian media quoted Mr. Medvedev's press secretary as saying that the call was initiated by the United States and the Russian prime minister stressed the need to "protect Ukrainian citizens, including in Crimea, as well as citizens of the Russian Federation located on the territory of Ukraine."
Meanwhile, Russia's Interfax news agency Monday quoted an unnamed official in the headquarters of Russia's Black Sea Fleet as denying reports that there are plans to attacks Ukrainian military units in Crimea, calling them "unmitigated nonsense."
Interfax had earlier Monday quoted unnamed officials in Ukraine's defense ministry as saying the commander of the Black Sea Fleet, Alexander Vitko, had given Ukrainian military forces in Crimea an ultimatum to surrender by 5 a.m., local time, on Tuesday, or face attack.
Russian Foreign Minister Sergei Lavrov said Monday that the use of Russian troops in Crimea is necessary "until the normalization of the political situation" on the peninsula. He spoke at the opening of the U.N. Human Rights Council session in Geneva and said his country's troops are protecting Russian nationals.
Lavrov met Monday with U.N. Secretary-General Ban Ki-moon, who is hoping to prevent a further escalation in the region.
Ukraine has accused Russia of carrying out a military invasion and urged Moscow to withdraw its forces, which have surrounded Ukrainian military bases across Crimea and set up roadblocks.
Russian Prime Minister Dmitry Medvedev signed a decree Monday ordering the start of preparations for building a bridge across the Kerch Strait connecting Crimea with Russia's Krasnodar region.
Russia's foreign ministry on Monday strongly criticized the Group of Eight major industrialized nations for suspending preparations for the G8 summit schedule to take place in the Russian city of Sochi in June over the events in Ukraine. The ministry said in a statement the move damages not only the G8 countries, but "the whole international community."
In a separate statement Monday, Russia's foreign ministry criticized what it called "unacceptable threats" made by U.S. Secretary of State John Kerry, who on Sunday called Russia's dispatch of troops to Crimea "a brazen act of aggression, in violation of international law."
The ministry accused Kerry of using "Cold War cliches" and the United States and its allies of "turning a blind eye" to what it called "the militant Russophobia and anti-Semitism" of the protesters who took power in Kyiv.
Russia's foreign ministry also says Russia and China have coinciding views on the situation in Ukraine, while the leaders of the G7 group of nations are condemning what they say is Russia's "clear violation" of Ukraine's sovereignty.
Lavrov spoke to his Chinese counterpart by phone on Monday. Chinese Foreign Ministry spokesman Qin Gang said China wants to see a political solution to the situation.
"We understand the historical background of the Ukraine issue, and the complexity of the current reality. As I have said yesterday, to get to this point today, things happened for a reason. We hope that all parties can, through dialogue and and consultation, find a political solution, prevent further escalation and work together to safeguard peace and stability in the region."
The G7 leaders issued a joint statement calling for Russia to address any concerns it has with Ukraine through either direct negotiations or mediation led by the United Nations or the Organization for Security and Cooperation in Europe (OSCE). The G7 includes the United States, Britain, Canada, France, Germany, Italy and Japan.
British Foreign Secretary William Hague is in Ukraine to meet with leaders, and on Monday visited Kyiv's Independence Square. He said in a BBC radio interview that Russian forces have taken "operational control of the Crimea" and that they must return to their bases there.
U.S. Secretary of State John Kerry is due to travel Tuesday to Ukraine.
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.