French President Francois Hollande and German Chancellor Angela Merkel are due to hold talks in Moscow to try to end escalating fighting in east Ukraine.
Their meeting with President Vladimir Putin follows urgent talks in Kiev, where a new peace plan was presented.
US Secretary of State John Kerry, who was also in Kiev, said the US wanted a diplomatic solution, but would not close its eyes to Russian aggression.
Moscow is accused of arming pro-Russian rebels - a claim it denies.
Russia also rejects claims by Ukraine and the West that its regular troops are fighting alongside the rebels against Ukrainian troops in the eastern Donetsk and Luhansk regions.
Clashes - which have intensified in recent weeks after a rebel offensive - have left nearly 5,400 people dead since April, the UN says.
As a result, a September ceasefire - signed in Minsk, Belarus - is now in tatters.