"I will look at all additional options that are available to us short of military confrontation and try to address this issue. And we will be in close consultation with our international partners, particularly European partners."
NATO accuses Moscow of sending troops to fight on behalf of rebels in territory the Kremlin has dubbed "New Russia" in a war that has killed more than 5,000 people.
In some of the strongest language ever from Brussels, Donald Tusk, the former Polish prime minister who now presides over EU summits as European Council president, denounced "appeasement" of Moscow, a word with unmistakable World War Two connotations.
"Once again, appeasement encourages the aggressor to greater acts of violence. Time to step up our policy based on cold facts, not illusions," Tusk said on Twitter.
Fighting in eastern Ukraine had mainly died down since a September ceasefire, but in recent days the war has returned in full force, with the rebels announcing the effective end of the truce and an offensive to expand territory under their control.
On Saturday rebels attacked Mariupol, a strategic Black Sea port of 500,000 people and the biggest city still in government hands in the two rebel-dominated eastern provinces. Kiev said 30 civilians were killed in shelling.
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Rebels launched new attacks on Sunday against government positions elsewhere along the front line that winds through the two restive provinces, the Kiev army said.
"Rebels are attacking the positions of anti-terrorist operation troops extremely intensively, using artillery, mortars, grenade launchers, tanks," military spokesman Andriy Lysenko said in a televised briefing.
He said four Ukrainian servicemen had been killed and 17 injured in the past 24 hours and that rebel attacks on the town of Debaltseve, northeast of separatist-held Donetsk, had been particularly fierce.
"Because of constant shelling in the past few days, there are dead and injured among local residents. Around 60 homes have been destroyed or damaged," he said without giving a figure for the number of casualties.
Rebel leader Alexander Zakharchenko said on Saturday the separatists planned to encircle Debaltseve, which has a population of around 26,000.