Ukraine's acting president has warned Russia that troop movements outside its naval base in Crimea would be seen as an "act of aggression". Armed men seized regional government headquarters and parliament and the Russian flag is flying from buildings
The latest challenges to attempts to set up a new national government in the capital Kiev were discussed by Mr Cameron and German chancellor Angela Merkel at talks in 10 Downing Street.
Ms Merkel said: 'We will do everything we can in order to support this government that has a tremendously difficult economic situation".
"The issue of the territorial integrity of Ukraine is of central importance to us".
Ukraine's interim President has expressed concerns about signs of separatism as opposition in Russian-speaking regions grows.
Hundreds of Ukrainians and journalists flocked to the presidential palace, as reports emerged that President Yanukovich had left the city.
Vitali Klitschko was met with boos from anti-government protesters, as he attempted to convince them a peace deal is best course of action.