Barack Obama has told his Russian counterpart Vladimir Putin that he must recognise the new Ukrainian President, Petro Poroshenko in order to help de-escalate the crisis in eastern Ukraine.
According to a White House official, the US President also told Mr Putin that if Russia works with the new Ukrainian authorities there could be "openings" to reduce tensions in the region.
Deputy national security adviser Ben Rhodes added that Mr Obama had demanded "ceasing support for separatists in eastern Ukraine, and stopping the provision of arms and material across the border".
David Cameron has issued a fresh warning to Russia over Ukraine ahead of meeting president Putin tomorrow.
Speaking from Brussels at the first G7 summit without Russia for 17 years, Mr Cameron said: "The G7 should send a clear message of support to Ukraine and a united message to President Putin that he needs to engage with the Ukrainian government to find a diplomatic solution to the crisis.
"That's what I'll be saying to President Putin myself tomorrow. This is the first summit without Russia since the 1990s. And until they change course, they need to understand that they will face continuing isolation and no seat at the table."
The G7 leaders effectively expelled Russia from the annual gathering, which Mr Putin had been due to to host in Sochi, following its incursion into Crimea.
Cameron and Putin are due to meet tomorrow in Paris ahead of the D-Day 70th anniversary commemorations on Friday.
Prime Minister David Cameron will meet Russian president Vladimir Putin on Friday for face-to-face talks on the crisis in Ukraine, Downing Street said.
Russian President Vladimir Putin has called for an 'immediate halt' to fighting in the eastern Ukrainian city of Donetsk.
In his first comments since new Ukraine president Petro Poroshenko was elected on Sunday Putin urged Kiev to start talks with rebel leaders.
But the Ukrainian government has accused Russia of sending "terrorists" across its border.
Ukraine's foreign ministry said: "There are grounds to believe that Russian terrorists are being sent onto Ukrainian territory, organised and financed under the direct control of the Kremlin and Russian special forces."
Russian President Vladimir Putin has dismissed talk of a new Cold War over the Ukraine crisis.
The former KGB agent blamed the unrest in the country on the West and said sanctions on the US and European Union would rebound.
He also denied trying to rebuild the Soviet Union after reclaiming Crimea and pledged he would work with whoever is elected in Ukraine this weekend.
"I would not like to think this is the start of a new Cold War. It is in no one's interest and I think it will not happen," he said.
The crisis has plunged East-West relations to their lowest level since the Cold War ended in 1991.
Vladimir Putin has said he is concerned that "radicals" in Ukraine may try to disrupt the supply of natural gas from Russia to Europe.
"I won't deny that we are seriously worried by Ukrainian radicals' statements and open threats to hinder gas transit. We still hope common sense will prevail there," the Russian president said.
Many European nations depend on Russian gas that flows through pipelines in Ukraine.
Russian President Vladimir Putin turned salesman today on a high-profile trip to China, where he was trying to secure a £240 billion gas deal.
After the Ukraine crisis, the President wants to find new markets for Russian gas.
ITV News China Correspondent Lucy Watson reports.
Russian President Vladimir Putin has taken part in an ice hockey match between Russian amateur players and ice hockey stars, at a festival of Russia's amateur hockey.
Donned in a red outfit and pads, the event was organised by the Night Hockey League near the Olympic Park in the Black Sea resort of Sochi, Russia.
The Obama administration has attacked Vladimir Putin over his visit to Crimea today, calling the move "provocative and unnecessary".
The Russian President visited the peninsula, annexed from Ukraine in March, to mark the anniversary of the Soviet Union's victory over Nazi Germany.
US State Department spokeswoman Jen Psaki issued a strong condemnation of the visit, saying:
"This trip is provocative and unnecessary. Crimea belongs to Ukraine and we don't recognize, of course, the illegal and illegitimate steps by Russia in that regard,"
European Ambassadors have agreed in principle to add approximately 15 people and five Crimean-based companies to the Russia sanctions list, diplomats told Reuters.