Russia denied new accusations by Kiev that it had sent soldiers and weapons to east Ukraine and held out hope of progress at talks on the conflict in Berlin despite renewed fighting.
Kiev accused Russian regular forces of attacking its troops in eastern Ukraine on Tuesday, one of its boldest assertions yet of direct Russian military involvement in the conflict between pro-Russian separatists and Ukrainian government forces.
Russian Foreign Minister Sergei Lavrov said Moscow had seen no evidence to back up such accusations and others made in the past few months.
"I say every time: if you allege this so confidently, present the facts. But nobody can present the facts, or doesn't want to - which it is, I don't know," he told a news conference when asked about the accusations.
"So before demanding from us that we stop doing something, please present proof that we have done it."
Lavrov did, however, acknowledge that the pro-Russian separatists in eastern Ukraine hold more territory than assigned to them under a ceasefire agreement reached last September.
He said Russia had received assurances from the separatists that they would retreat to the separation lines agreed last September in the Belarussian capital, Minsk.
Kiev accused the separatists on Tuesday of seizing more than 500 square km (194 square miles) of territory beyond the agreed separation lines.
Lavrov said the conflict in east Ukraine had to be solved with respect for the country's integrity, underlining Moscow's support for the two separatist-controlled regions in the east to remain in Ukraine but with more autonomy.
He expressed hope that talks in Berlin later on Wednesday involving Lavrov, the Ukrainian, French and German foreign ministers would make progress towards holding a summit of their countries' leaders on the crisis.
"We hope the contacts to be held in a foreseeable future at various levels and in various formats will help move ahead in this direction," he said.