PARIS  - The West expressed alarm on Saturday over fast-moving developments in Ukraine's Crimea, urging all sides to avoid further escalation and calling on Russia to respect Ukraine's sovereignty.

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".

British Prime Minister David Cameron said he had told Russian President Vladimir Putin in a phone call on Friday that

"there can be no excuse for outside military intervention in Ukraine".

"Everyone must think carefully about their actions and work to lower, not escalate, tension," he said. "The world is watching."

French President Francois Hollande spoke to Putin on Saturday to tell him of his "grave concern" and urge him to "avoid any use of force and to seek a solution to the crisis with the international community", a statement from Hollande's office said.

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," his office said.

Hollande also spoke about the situation in Crimea with U.S. President Barack Obama, German Chancellor Angela Merkel and European President Herman Van Rompuy.

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.

Putin 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.

German Foreign Minister Frank-Walter Steinmeier called developments in Crimea 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.