Germany's Foreign Minister Frank-Walter Steinmeier has called on all sides to respect the ceasefire in eastern Ukraine amid rising tensions.
"Unfortunately, the situation is again serious. I now think that we must call upon all those involved in this conflict, as well as the Russian side, to return again to the terms of the Minsk protocol," he said.
The fragile peace in the Ukraine's east has looked increasingly perilous in recent days.
The rebel held Donetsk was shelled on Sunday, while the Organisation for Security and Cooperation in Europe said it had seen an armoured column of troops in rebel territory. Kiev said this proved its claim that Russia had sent additional reinforcements to the front line.
Ukraine's central government sent reinforcements to the country's south and east last week, saying it was preparing for an advance from the separatist rebel fighters.
Both sides have claimed that the other is responsible for breaking the terms of the peace agreement signed in the Belarussian capital of Minsk in September.
The escalation comes after two regions under the control of pro-Russian rebels, Luhansk and Donetsk, held rogue elections in which rebel candidates claimed victory.
Ukraine and its allies denounced the results, while Russia endorsed the outcomes of the rogue polls.
Russia has denied sending reinforcements to eastern Ukraine.