The Ukrainian military and rebels started an agreed prisoner swap in a frontline town in the country's east on Saturday. The rebels said they were exchanging more than 130 government troops for around 50 of their fighters.

The exchange was being carried out in the frontline town of Zholobok, some 40 kilometers (about 25 miles) northwest of the rebel stronghold of Luhansk, according to the AFP news agency.

The move was part of a UN-backed truce agreed in the Belarusian capital, Minsk, which came into effect a week ago.

Ukraine's President Petro Poroshenko confirmed on Twitter that soldiers held captive by the rebels were being released:

Poroshenko said he had been informed by Ukraine's national security service that the process had begun. But he did not specify where the release was taking place or how many captive rebels would be exchanged altogether.

Moscow warned once again by Washington

The US said on Saturday that sanctions against Russia could be tightened after separatists took over the key eastern Ukrainian town of Debaltseve on Wednesday in violation of the recent Minsk peace agreement.

"We're not going to sit there and be part of this kind of extraordinarily craven behavior at the expense of the sovereignty and integrity of a nation," US Secretary of State John Kerry said in London before talks with British counterpart Philip Hammond.

The peace accord stated that both sides had to withdraw heavy weaponry from the front line by March 3, carry out a prisoner exchange, negotiate over greater autonomy in rebel-held areas and eventually restore Ukraine's control of its border with Russia.

German Chancellor Angela Merkel held talks with French President Francois Hollande in Paris on Friday

Germany and France, which helped broker the truce, said they were determined it would be implemented despite it being constantly violated.

"We don't have any illusions" about the difficulty involved, German Chancellor Angela Merkel said after meeting French President Francois Hollande in Paris on Friday.

But she said she was "even more convinced" a negotiated solution was the only way to end the conflict.

The UN said around 5,700 people had been killed since the conflict began in April last year.

lw/bk (AP, AFP, Reuters)