Obama calls Cameron to discuss Ukraine

Both leaders came to an agreement to implement further sanctions on Russia

US President Barack Obama has telephoned British Prime Minister David Cameron and agreed to implement further sanctions on Russia unless it takes firm measures to deescalate the situation in Ukraine.

Obama and Cameron welcomed the news that separatists have agreed to implement the ceasefire declared unilaterally by Ukrainian President Petro Poroshenko, the statement said.

The two leaders re-emphasised the need for Russia to stop the flow of weapons and militants across the border and to use its influence over separatist groups to encourage them to take immediate concrete steps to implement Poroshenko's peace plan.

"They agreed that should Russia fail to take these immediate steps to deescalate the situation in eastern Ukraine, the United States and the European Union would work to implement additional coordinated measures to impose costs on Russia," the White House said.

Obama and Cameron also discussed the situation in Iraq and the threat posed by ISIL.

Separately, US Vice President Joe Biden spoke with Poroshenko and offered condolences for the deaths of Ukrainian service members, including the shooting down of a Ukrainian transport helicopter in eastern Ukraine.

Biden underscored the importance of having monitors in place to verify violations of the ceasefire, as well as the need to stop the supply of weapons and militants from across the border, the White House said in a separate statement.

Later White House Press Secretary Josh Earnest told reporters that the US welcomes any Russian steps to end the crisis in Ukraine, including the Russian President, Vladimir Putin's, request to the Duma to revoke a resolution authorising the use of Russian military forces in Ukraine.