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"Direct military aggression by the Russian Federation in the east of Ukraine is continuing. The Russians are continuing to send military equipment and 'mercenaries'," Ukraine's defense and security council said in a separate Twitter post.

Kiev and Western countries say recent rebel gains were the result of the arrival of armored columns of Russian troops, sent by Russian President Vladimir Putin to prop up a separatist rebellion that would otherwise have been near collapse.

There was no immediate fresh comment from Russia on Saturday. Putin on Friday compared Kiev's drive to regain control of its rebellious eastern cities to the Nazi invasion of the Soviet Union in World War Two.

'No time to waste'

EU leaders meeting in Brussels appointed Polish Prime Minister Donald Tusk as president of the European Council, giving hawkish Kremlin critics in Eastern Europe new influence in the bloc.

According to a draft statement from the summit, the EU leaders were set to ask the European Commission and the EU's diplomatic service "to urgently undertake preparatory work" on further sanctions that could be implemented if necessary.

Read MoreUkraine: Why youshould really be worried this time

French President Francois Hollande stressed that a failure by Russia to reverse a flow of weapons and troops into eastern Ukraine would force the bloc to impose new economic measures.

"Are we going to let the situation worsen, until it leads to war?" Hollande said at a news conference. "Because that's the risk today. There is no time to waste."

European Commission President Jose Manuel Barroso said the EU was prepared to toughen sanctions against Russia but also that it wanted a political deal to end the confrontation.

Continued here:
Ukraine: Close to point of all-out war with Russia