A week after violent protests forced Russian-backed President Viktor Yanukovich to abandon power in Kiev, Ukraine's new leaders say Russia is trying to take control of the southern Crimea region, which has a majority ethnic Russian population.
France, Britain and Germany issued calls for de-escalation in Crimea hours after U.S. President Barack Obama warned that military intervention in the region would be deeply destabilising and "carry costs".
"France is extremely concerned by the reports from Crimea, which describe significant troop movements," French Foreign Minister Laurent Fabius said in a statement.
"We call on the parties to abstain from acts that could raise tensions and affect Ukraine's territorial unity."
In a statement, French President Francois Hollande urged European countries to take swift and decisive action to find a way out of the crisis in Crimea when their foreign ministers meet in Brussels on Monday.
"Everything must be done to avoid outside intervention and the risk of a highly dangerous escalation," Hollande's office said in a statement.
Hollande had spoken to Polish Prime Minister Donald Tusk and both expressed alarm over situation, which the statement said posed a "real threat to Ukraine's territorial unity and sovereignty".
No bloodshed followed Yanukovich's overthrow, but Ukraine's new leadership faces a challenge in Crimea, which was part of Russia until 1954.
APPEALS TO PUTIN
Ukrainian Prime Minister Arseny Yatseniuk accused Russia on Saturday of sending thousands of troops to the area.
Armed men wearing combat uniforms with no identification have taken control of two airports in the area and have taken over the regional parliament in what Kiev describes as an occupation by Moscow's forces.
Crimea's pro-Russia prime minister has put himself in charge of all military forces, police and other security services in the region. He has also appealed to Russian President Vladimir Putin for assistance in "guaranteeing peace and calm" there.
Putin has obtained authorisation from the upper house of parliament to send armed forces to Ukraine's Crimea region, although a spokesman said that no decision had yet been taken on whether to send them.
British Foreign Secretary William Hague, who travels to Ukraine on Sunday to hold talks with the new leadership, urged his Russian counterpart to act to ease tensions and said Russia was posing a potentially grave threat to Ukraine.
Hague said Britain supported the Ukrainian government's request for urgent consultations in accordance with the 1994 Budapest Memorandum, signed by Britain, United States, Russia and Ukraine.
The memorandum provided guarantees of Ukraine's sovereignty and integrity in exchange for a Ukrainian commitment, since fulfilled, to give up its Soviet-era nuclear weapons.
Prime Minister David Cameron said Britain had called an emergency meeting of the United Nations Security Council in view of the "serious and concerning events" in Ukraine.
"There can be no excuse for outside military intervention in Ukraine - a point I made to President Putin when we spoke yesterday," Cameron said.
"Everyone must think carefully about their actions and work to lower, not escalate tension. The world is watching."
German Foreign Minister Frank-Walter Steinmeier called developments in Crimea over the past few hours dangerous and urged Russia to explain its intentions.
"The situation in Crimea in particular has become considerably more acute. Whoever pours more oil onto the flames now, with words or actions, is consciously aiming for further escalation of the situation," he said.
"Everything Russia does in Crimea must be in keeping with the sovereignty and territorial integrity of Ukraine, and treaties on Russia's Black Sea fleet."
Steinmeier said European leaders must confer swiftly in order to agree a common position of the European Union.
Russia says any movements by its military in Crimea are in line with agreements with Ukraine in the lease of a naval base in the port city of Sevastopol, and Moscow has accused Kiev of trying to destabilise the Black Sea peninsula.