It has been a day of intense diplomatic efforts to resolve the crisis in Ukraine with the US and Russia exchanging strongly-worded messages.

The US Secretary of State John Kerry urged the Russians to abide by international law and accused them of "acts of aggression".

But President Putin has defended his intervention insisting on the right to use force but only, he said, as a "last resort".

The tension has been highest in Crimea where there was a momentary confrontation between Russian and Ukrainian troops at a military airbase.

John Simpson reports from Sevastopol.