"We hope that the declarations by Russian politicians that the troops are to be withdrawn from Ukraine's borders don't just remain declarations," he said in Berlin.
"At this point we cannot confirm it," he said, speaking through an interpreter at a meeting with German counterpart Frank-Walter Steinmeier.
Ukraine's border guards said earlier that Russian forces had left the area within 10 kilometres of the border after Russian President Vladimir Putin ordered their withdrawal.
If confirmed, the withdrawal would ease tensions five days ahead of a make-or-break Ukraine presidential poll.
"It could be the key to bringing back economic and political stability. We hope that these elections will be the beginning of a stabilisation of Ukraine.