US Secretary of State John Kerry has called on Russia to show "within hours" it is working to disarm separatist militants in eastern Ukraine.
He was speaking to reporters in Paris a day before a shaky ceasefire was due to end in the Donetsk and Luhansk regions.
Russia denies it has let militants and heavy weaponry cross its border into eastern Ukraine.
The US and EU are threatening to impose further sanctions if Russia does not act to defuse the situation.
Separatist leaders in Donetsk and Luhansk have declared independence and the West accuses Russia of fomenting unrest.
Despite the ceasefire, the rebels shot down a helicopter near Sloviansk, Donetsk region, on Tuesday with the loss of nine lives. They have accused the government of not honouring its own truce, saying Ukrainian forces have bombarded local villages with artillery and mortars.
Talks on extending the ceasefire are set to take place in Donetsk on Friday, the rebels have confirmed.
On the same day, Ukraine's President Petro Poroshenko is set to sign a far-reaching free trade deal in Brussels as part of a landmark EU association agreement.
"It is critical for Russia to show in the next hours, literally, that they are moving to help disarm the separatists, to encourage them to disarm, to call on them to lay down their weapons and begin to become part of a legitimate political process," Mr Kerry told reporters after talks with French Foreign Minister Laurent Fabius.
Mr Poroshenko, speaking in Strasbourg, echoed his words, urging Russian President Vladimir Putin to back "with deeds, not words" his 15-point plan to bring peace to the region. So far, Russia's support had been "insufficient", he said.
On the peace talks, he was quoted by Ukrainian media as saying: "If our conditions for the peace plan are not accepted, then we will make a very important decision."
However, the proposals are not thought to go far enough for many of the separatist leaders.
Mr Putin held lengthy talks by phone with the leaders of Ukraine, Germany and France on Wednesday.
He then spoke to Germany's Chancellor Angela Merkel again on Thursday, ahead of a key European Union summit. According to the Kremlin, the pair discussed the "need to extend the ceasefire".
Mr Putin also discussed the humanitarian situation in eastern Ukraine where some areas have no power or water. Thousands of people are believed to have fled their homes, with more than 14,000 seeking refuge across the border, according to the Russian government.
The crisis in Ukraine dates back to last November, when President Viktor Yanukovych decided not to sign the agreement at the last minute, opting instead for closer ties with Moscow.
After months of protests, he eventually fled the country and was deposed as president.
Russia then moved to annex Ukraine's Crimea region and pro-Russia separatists took over key buildings in dozens of eastern cities, claiming that extremists had taken power in Kiev.
More than 420 people have been killed in violence in the east since mid-April, the UN estimates.