The Ukrainian government and pro-Russian rebels have exchanged prisoners in the east of the country as part of the shaky international cease-fire reached nearly two weeks ago.
One hundred and thirty-nine Ukrainian troops and 52 rebels were exchanged late on February 21 some 20 kilometers west of Luhansk, according to both sides.
The exchange is a key part of the deal between the leaders of France, Germany, Russia, and Ukraine on February 12.
U.S. Secretary of State John Kerry said the United States and its European allies are in talks about harsher sanctions against Moscow.
Kerry said U.S. President Barack Obama will evaluate next steps in dealing with the conflict in eastern Ukraine in the coming days.
Kerry said the most "egregious violation" of the cease-fire was the assault on the city of Debaltseve and military supplies sent by Moscow to separatists.
Kremlin spokesman Dmitry Peskov said Russia was focused on supporting the cease-fire deal.
Earlier, the Ukrainian prisoners were transported to a location near Zholobok, where they were made to line up and listen to a speech by a rebel representative, who ordered the men to leave the territory claimed by the separatist movements in the regions of Donetsk and Luhansk.
The soldiers -- some of them using crutches, at least one carried on a stretcher -- then walked for around 3 kilometers to a rendezvous point.
At talks in London, Kerry accused Russia and the rebels of complying with the cease-fire accords in a few areas.
"We know to a certainty what Russia has been providing and no amount of propaganda is capable of hiding these actions," Kerry said.
He said among options being considered by Obama was whether to arm Ukrainian forces and to impose deeper sanctions against the Kremlin.
Kerry pointed to the strategic port of Mariupol, which Ukrainian forces say is threatened by a buildup of rebel military equipment.
Speaking to Echo Moskvy radio station, Peskov warned against more sanctions against Russia.
"An obsessive idea to force someone to pay the cost...is not conducive to the resolution of the situation in southeast Ukraine," Peskov was quoted as saying.
Ukraine's military on February 21 reported attacks over the past day that killed a serviceman and wounded 40, with mortars reaching the fringes of Mariupol, among other places.
According to Reuters, some rebels had formed a base in a complex of houses in Bezimenne further up the coast from Mariupol.
Reuters reporters said there were dozens of well-armed fighters milling around, some of whom looked like Russian military special forces wearing Russian army patches and insignia on their uniforms.
In Bezimenne, one rebel fighter who gave his nom de guerre as Boxer denied the Kyiv reports of more Russian tanks and fighters being sent to the area.
"It's all a lie. The only people fighting here are miners, tractor drivers, and farm workers," he said.
Moscow has been supporting separatism in eastern Ukraine since street unrest chased pro-Russian President Viktor Yanukovych from Kyiv one year ago, including supplying arms and troops to rebels fighting a conflict that has so far killed more than 5,400 people.
Russia accuses the West of orchestrating the so-called Maidan unrest, which was sparked by a decision to spurn closer ties with the European Union.