David Cameron has warned Vladimir Putin of "far-reaching sanctions" against Moscow if Ukraine is further destabilised as the two leaders agreed to "work closely" to defuse the issue.
Downing Street said the Prime Minister telephoned the Russian president to discuss the latest situation and push for the implementation of the peace accord agreed in Geneva.
A new round of sanctions imposed by the European Union and targeting senior Russian officials and politicians came into force yesterday, as the West responded to continued unrest in the east of Ukraine.
A Number 10 spokesman said: " The Prime Minister underlined the importance of finding a means to de-escalate the situation ahead of Ukraine's presidential election on May 25, and his view that the Geneva process remained the best way of achieving this.
"The Prime Minister re-iterated our commitment both to the implementation of the Geneva Accord in partnership with the Ukrainian and Russian governments, and to all Ukrainians in being able to participate in free and fair democratic elections.
"The Prime Minister noted that further serious destabilisation in Ukraine would inevitably result in the imposition of far-reaching sanctions by the G7 and European Union
"The Prime Minister and President Putin agreed that their governments should continue to work closely to find ways of de-escalating the current situation and helping restore stability in Ukraine."
The call came as it was announced that Britain is to send a prosecutor to Ukraine to help local law enforcement authorities in the hunt for billions of pounds believed to have been siphoned out of the country by ousted former president Viktor Yanukovych and his inner circle.
The announcement was made by Attorney General Dominic Grieve at the end of a conference in London which brought together anti-corruption experts from more than 30 countries and international agencies to co-ordinate efforts to retrieve stolen assets which Ukrainian general prosecutor Oleh Makhnitskyi said could run into tens of billions of US dollars.
The UK prosecutor will work with a team of financial investigation experts from the National Crime Agency who have already gone to Ukraine, while the Serious Fraud Office in Britain "stands ready to support Ukraine law enforcement and provide whatever practical and technical assistance on the ground is needed", said Mr Grieve.
The two-day Ukraine Forum on Asset Recovery (UFAR), co-hosted by the UK and US at Lancaster House, heard Mr Makhnitskyi describe the Yanukovych regime as a mafia-style "organised criminal group" which had systematically misappropriated for their personal use funds belonging to the Ukrainian people.
Mr Grieve praised the "astonishing courage" of anti-corruption protesters whose demonstrations prompted Yanukovych's overthrow in February.
The London forum had heard evidence of the "shocking dishonesty and greed of ministers and public officials who held office under Yanukovych", said Mr Grieve.
"The full extent of that criminality is now being uncovered, with potentially billions of pounds stolen.
"Ukraine faces significant challenges to establish a transparent and legitimate system of government and to purge its public institutions of the legacies of corruption. The Ukrainian people will need to continue to show courage and resolve to achieve these goals in the face of external aggression. They will have the support of the international community to do so."
US attorney general Eric Holder said that the countries taking part in the UFAR conference had shown "renewed determination to stand shoulder-to-shoulder with Ukraine in every way in its time of challenge, just as we will stand with any nation embattled by kleptocracy and corruption."
He added: "I want to assure the world that as long as Ukraine, as long as any nation, has the courage to oppose corruption and to challenge exploitation, they will find steadfast friends and strong allies in every corner of the globe."
Mr Makhnitskyi said that Yanukovych and his cronies had done "everything possible" to get assets out of the country to boltholes abroad, and vowed: "We will do everything possible to return to the Ukrainian people all these stolen assets. We understand it is not an easy process, it is a complicated process that requires time and common effort."
Ukraine has sent information about the stolen assets to 136 countries, and has received co-operation from countries such as Austria and Liechtenstein, which are known to host numerous overseas bank accounts, while Swiss authorities have said they are aware of two billion US dollars worth of assets of Ukrainian origin, said Mr Makhnitskyi.
But he made clear that much of the illicit money is likely to be in Russia, where it may be difficult to recover.
"Russia now has these people such as Yanukovych," said Mr Makhnitskyi.
"We need those people out. The general prosecutor of Russia, Yury Chaika, has said Russia will not give them up. By our latest information, 32 billion US dollars were taken out of Ukraine to Russia."