Foreign secretary David Miliband is on his way to Ukraine as part of a bid to build what he described as "the widest possible coalition against Russian aggression".
He will deliver a major speech condemning Russia's decision to formally recognise two breakaway regions of Georgia.
Russian president Dmitry Medvedev has been accused of "inflaming" the crisis by insisting that South Ossetia and Abkhazia should be independent.
He also warned that he could authorise military force to respond to the controversial deployment of an American missile defence shield in Poland and the Czech Republic.
Mr Miliband said Russia's recognition of the two regions was "unjustifiable and unacceptable" and further inflamed an already tense situation in the region.
"It will also not work," he said in a statement. "It is contrary to the principles of the peace agreement, which Russia recently agreed, and to recent Russian statements.
