The BBC's James Reynolds reports from the government buildings in Donetsk, a focal point for tension. Photo: BBC NEWS

The Russian president told Mr Obama Moscow could not ignore calls for help from Russian speakers, but that ties with the US should not be affected.

Crimean MPs have asked to join Russia and called a referendum on the move.

The Ukrainian Paralympic team has confirmed it will not boycott the Sochi Winter Paralympics.

The head of the team, Valeriy Sushkevich, said they would participate "so they remember us, remember Ukraine - a sovereign state, which sent its athletes here".

But he said: "If something major happens, Ukraine will leave the Games immediately".

The Games, which open in the Russian Black Sea resort later on Friday, has already been boycotted by many foreign dignitaries amid international tensions over Russia's military intervention in the southern Ukrainian region of Crimea.