Russia's President Vladimir Putin said on Thursday that pro-Russian rebels had encircled up to 8,000 Ukrainian troops and expected them to surrender ahead of a ceasefire.

"Of course, they (rebels) proceed from the assumption that this group will lay down arms and stop putting up resistance," Putin said after marathon talks with the leaders of Germany, France and Ukraine, citing information from separatists.

Putin said pro-Russian rebels had encircled a "significant group of 6,000 to 8,000" Ukrainian troops.

Putin did not specify where the government troops had been encircled but he apparently referred to fighting near the town of Debaltseve, where Ukrainian soldiers have been under heavy rebel artillery fire.

"We are calling on all sides to show restraint" to avoid "unnecessary bloodletting that no one needs" ahead of the ceasefire set to start on February 15.

Putin indicated that the issue had apparently been a key sticking point in the talks.

"Initially I had doubts," Putin said. "I am ready to share them."

"The Ukrainian leadership believes that there's no encirclement and that's why they believe that everything will be rather smooth."

"If such an encirclement did take place then proceeding from normal logic, those who have been encircled, will make attempts to break out and those who are outside will make attempts to establish a corridor for their encircled fellow serviceman."

Putin said his Ukrainian counterpart Petro Poroshenko and he had tasked military experts to look closely into the issue.

Vladyslav Seleznyov, spokesman for Ukraine's General Staff, denied however that Ukrainians troops had been blocked near Debaltseve.

"The situation remains tense," he said, adding however that fighting in the area was continuing.