British Prime Minister David Cameron told President Vladimir Putin on Friday that Russia must respect the territorial integrity of Ukraine.
Cameron called Putin by telephone to discuss the escalating tension in Ukraine's Crimea region, a Downing Street spokesman said.
"The Prime Minister emphasised that all countries should respect the territorial integrity and sovereignty of Ukraine. President Putin agreed, stressing that Russian military exercises in the area had been planned before the current situation in Ukraine," the spokesman said.
"They agreed that the free and fair elections that the interim government has pledged to hold are the best way to secure a positive future for Ukraine."
Cameron and Putin agreed the international community should consider how to help Ukraine tackle its economic challenges, the spokesman said.