U.S. President Barack Obama has placed more telephone calls to European leaders to discuss Ukraine, and the White House said all agreed that Russian troops must return to their bases.
A statement issued Saturday says Mr. Obama spoke to British Prime Minister David Cameron, French President Francois Hollande, and Italian Prime Minister Matteo Renzi.
According to the statement, the leaders all say Russia is violating international law and agree on the need for international observers in Crimea. They rejected a proposed referendum on Crimea's future as a violation of Ukraine's constitution.
Mr. Obama also held a conference call with Estonian President Toomas Hendrik Ilves, Latvian President Andris Berzins, and Lithuanian President Dalia Grybauskaite. The White House said the Baltic leaders welcomed NATO's decision to step up patrols over Baltic airspace in response to the situation in Ukraine.
Meanwhile, the State Department says Secretary of State John Kerry made another telephone call to Russian Foreign Minister Sergey Lavrov.
A statement says Kerry told him that continued military escalation in Ukraine along with steps by Russia to annex Crimea would "close any available space" for diplomacy.
The Pentagon estimates that 20,000 Russian troops are in Ukraine.
President Obama has already imposed sanctions on those he says are found to have stolen assets of the Ukrainian people or violated Ukraine's territorial integrity. The sanctions include visa restrictions but do not name targeted individuals.