Foreign Secretary William Hague will join US counterpart John Kerry for talks on emergency support for Ukraine today as Moscow questioned the legitimacy of the country's interim leaders.
Mr Hague has warned that Ukraine faces imminent economic collapse without support from the international community and Washington has said it stands ready to plough in cash with other partners to stabilise the nation.
Ousted president Viktor Yanukovych has reportedly fled to a pro-Russian area in Ukraine and a warrant has been issued for his arrest.
Russian prime minister Dmitry Medvedev said authorities in the country had taken power as a result of "armed mutiny".
"If you consider Kalashnikov-toting people in black masks who are roaming Kiev to be the government, then it will be hard for us to work with that government," he said.
During his visit to Washington, Mr Hague will discuss the country's financial plight with the International Monetary Fund.
"Ukraine's financial situation is very serious and without outside assistance may not be sustainable," he told MPs yesterday. "An economic crisis in Ukraine would be a grave threat to the country's stability and have damaging wider consequences."
While support could be provided quickly once it was requested by a new government in Kiev, Mr Hague said it would require a commitment to reform.
"It requires a stable and legitimate government to be in place and a commitment to the reforms necessary to produce economic stability. International financial support cannot be provided without conditions and clarity that it will be put to proper use," he said.
Mr Hague, who discussed the issue by telephone with Russian foreign minister Sergey Lavrov, and stressed it was " important that all channels of communication between the EU and Russia stay open".
Despite tensions between western Ukraine, where opposition to Mr Yanukovych was strongest, and the Russian-speaking east, which traditionally looks more to Moscow, Mr Hague said Mr Lavrov had emphasised Russia's commitment to the territorial integrity of Ukraine.
He told MPs that the Russian foreign minister had not raised any prospect of military intervention by the Kremlin in the country.
"Any notion of this kind is manifestly not in the interests of Russia, as well as not in the interests of Ukraine. I hope that point is well understood," he said.