United States President Barack Obama has called British Prime Minister David Cameroon to consult on the unfolding developments in Ukraine as both the sides have agreed that Russia's action violated Ukrainian sovereignty.

During the call, Obama and Cameroon expressed their grave concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, the White House said in a statement yesterday.

"Both noted that the current circumstances are unacceptable. Russia has already started to pay a cost for its actions, such as reducing investor confidence in Russia," it said.

"The two leaders welcomed the military observer mission being undertaken by Organisation for Security and Cooperation in Europe at the request of the Government of Ukraine," the statement said adding that they also discussed support for the government of Ukraine as it works to stabilise its economy and makes preparations for elections in May.