SEOUL (AFP) - United States President Barack Obama said he would consult key European leaders later on Friday on the failed agreement to ease tensions in Ukraine and on imposing new sanctions against Russia.

Mr Obama said in Seoul that he wanted to coordinate a united approach with Western United States allies, after Washington concluded that Russia had done nothing to implement the deal reached in Geneva last week.

"I will be talking to the Europeans, not all of them, but some key European leaders again this evening, making sure they share my assessment in terms of what has happened since the Geneva talks," Mr Obama said at a press conference in Seoul.

The President said Washington had already lined up further targeted sanctions against Russia "that are ready to go".

A senior US official said the conference call would include British Prime Minister David Cameron, German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minister Matteo Renzi.

Mr Obama also signalled that the new sanctions, widely expected to be imposed on Russia within days, would not involve an attempt to target key areas of the Russian economy, including the mining and energy sectors.

US officials have said those measures would only be considered if Russia sent its regular forces across the border into eastern Ukraine.

"We will continue to keep some arrows in our quiver," Mr Obama said.

The coming toughened sanctions could bite deeper into Russian President Vladimir Putin's inner circle by hitting prominent Russian businessmen tied to Mr Putin with travel and visa bans.

Earlier, in Washington, Secretary of State John Kerry warned Russia not to make an "expensive" mistake, in what US officials said was a last appeal to Russia to implement the Geneva deal reached between Washington, Moscow, Ukraine and the European Union.

He said Moscow had sabotaged the deal by supporting pro-separatist militias inside eastern Ukraine - weeks after annexing Crimea.

Mr Obama will conduct the conference call with European allies from the road in South Korea on the second leg of a four-nation Asian tour overshadowed by the worst East-West showdown since the Cold War.

The President has faced intense political pressure at home to move quickly to add to the sanctions already imposed against key allies of Mr Putin and a bank linked to Moscow's political elite.

Washington wants to move in lock step with Europe in the knowledge that the continent has far greater investments and economic ties with Russia than the US does.

Despite whispers that some US officials are tiring of Europe's reluctance to levy additional sanctions, Mr Obama praised the response of US allies to the Ukraine crisis.

"They have been unequivocal in condemning Russia," Mr Obama said.

"They have moved steadily when it comes to applying sanctions and consequences towards Russia.

"But there are a lot of countries inside Europe - they have a whole process they have to go through," Mr Obama said.