US President Barack Obama and his European allies expressed "grave concern" over Russia's violation of Ukrainian sovereignty and territorial integrity, terming it a threat to peace and security.

President Obama spoke separately with British Prime Minister David Cameron, Polish President Bronislaw Komorowski and German Chancellor Angela Merkel last afternoon and discussed the Ukrainian crisis.

"The leaders expressed their grave concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of law and a threat to peace and security," the White House said in a statement yesterday.

They stressed that the dialogue between Ukraine and Russia should start immediately, with international facilitation as appropriate, it said.

The leaders affirmed the importance of unity within the international community in support of international law and their support for the Ukrainian government, including its territorial integrity and its efforts to move forward with elections in May so that the Ukrainian people can continue to determine their own future in this historic hour.

According to the White House, leaders pledged to work together on a package of multilateral and bilateral financial assistance to help Ukraine as it pursues urgently needed reforms to stabilise its economy.

"The leaders agreed to continue to coordinate closely, including bilaterally, and through appropriate international organisations," the statement said.

Earlier, President Obama had warned his Russian counterpart Vladimir Putin against violation of Ukrainian sovereignty and asked him to pull the forces back to their bases in Crimea or face political and economic isolation.