President Barack Obama and key European allies have expressed "grave concern" over Russia's actions in Ukraine as Washington warned further military escalation could jeopardise chances of brokering a diplomatic solution to the crisis.
The White House said Obama discussed Ukraine in calls with French counterpart Francois Hollande, British Prime Minister David Cameron and Italian leader Matteo Renzi.
"The leaders reiterated their grave concern over Russia's clear violation of international law and reaffirmed their support for Ukraine's sovereignty and territorial integrity," a White House summary of the calls said.
Obama also hosted a conference call with the leaders of Baltic former Soviet states Lithuania, Latvia and Estonia to discuss the ongoing situation.
In a separate development, US Secretary of State John Kerry held a phone conversation with Russian Foreign Minister Sergei Lavrov.
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A State Department official said Kerry emphasised Washington was working to "facilitate direct dialogue between Ukraine and Russia".
"At the same time, he made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia would close any available space for diplomacy," the State Department said. "(Kerry) urged utmost restraint."
Earlier Saturday, the French presidency said in Paris that Hollande and Obama discussed "new measures" against Russia if it failed to act to defuse the crisis.
Hollande and Obama insisted on the "need for Russia to withdraw forces sent to Crimea since the end of February and to do everything to allow the deployment of international observers", Hollande's office said.
"If there's a lack of progress in this direction, new measures will be taken which would noticeably affect relations between the international community and Russia, which is in no one's interest."
Unrest in Ukraine, which resulted in the ouster last month of pro-Russian president Viktor Yanukovych, has worsened since the Crimean parliament's decision Thursday to secede and stage a March 16 referendum on joining Russia.
Ukraine also said there were now 30,000 Russian soldiers in Crimea - 5000 more than allowed under an existing agreement with Kiev.
Russia says it has stepped up protection of its naval base on the peninsula and is working together with local self-defence units but refuses to acknowledge deploying extra troops.
