Obama targets putins allies
NATO shakes up Russia strategy over Ukraine crisis
British PM David Cameron said pressure on Moscow would mount if it did not curtail military action.
The Russian President said he believed Kiev and pro-Russian separatists could come to an agreement.
An import ban on food comes after Vladimir Putin ordered retaliation for Western sanctions against Moscow.
The rebels show no sign of giving up their arms, but opening talks would be a big step on the road to peace.
G7 leaders threatened to impose harder sanctions on Russia over stability in eastern Ukraine.
Ukrainian forces launched an operation to retake the eastern town of Slaviansk on Thursday.
The sources said Obama was expected to speak in a conference call on Friday with British Prime Minister.
Vladimir Putin aims to erect a Eurasian Union of former Soviet states
The US and EU agreed to work together to prepare possible tougher economic sanctions.
Earlier on Monday, Russian troops forced their way into a Ukrainian marine base.
Barack Obama will seek support for his firm line at a meeting with other leaders of the G7.
President Vladimir Putin signed laws completing Russia's annexation of Crimea on Friday.
Barack Obama raised the stakes in an East-West confrontation over Crimea.
Russia's foreign ministry has responded saying the action would 'hit the US like a boomerang'.