KIEV, Ukraine — A prisoner exchange started Saturday in eastern Ukraine between pro-Russian rebels and the Ukrainian government.

The exchange included the freeing of some 140 Ukrainian soldiers who have been held captive by the rebels, Ukrainian President Petro Poroshenko said.

Poroshenko wrote on his Twitter account that he was informed the process had begun by the national security service.

“In the near time, 140 of our heroes will be free,” he wrote, without specifying further.

An Associated Press journalist who witnessed the exchange in the Luhansk region in eastern Ukraine was told by a rebel official that it involved 139 Ukrainian soldiers and 52 rebels.

The peace agreement signed last week in Kiev foresees an exchange of all prisoners in the conflict. It is unclear how many are held in total on both sides, although the Donetsk separatists have said Ukraine is holding about 580 rebels as prisoners.

Elsewhere Saturday, Ukraine’s military and the Russia-backed separatist rebels accused each other of continuing to mount attacks a week after a cease-fire was called.

Ukrainian security spokesman Col. Andriy Lysenko said that one serviceman was killed and 40 wounded in attacks over the past day. He did not state a total number of attacks, but said there were 10 mortar attacks on Ukrainian forces in the village of Shyrokyne on the fringes of the strategic port city of Mariupol.

Lysenko said rebels continued to move equipment toward Mariupol. Concerns persist that rebels aim to seize the city which would aid establishing a land corridor between mainland Russia and the Russia-annexed Crimea peninsula.

The rebels claimed Ukrainian forces launched 15 shelling attacks overnight, including on parts of Donetsk, the largest rebel-controlled city.

An agreement reached by the leaders of Ukraine, Russia, Germany and France called for the guns to go quiet on Sunday.

The warring sides were supposed to begin drawing back heavy weapons from the front lines on Tuesday, but international monitors say they’ve seen no sign of that yet.

Russian and Ukrainian military officials overseeing the hoped-for peace process announced Friday that the Ukrainian government and the rebels had worked out a plan to begin the weapons pull-out.

Heidi Tagliavini, an envoy for the Organization for Security and Cooperation in Europe who led the talks with Russia and Ukraine that also included rebel figures, remained cautiously hopeful.

“There is not a single day in the Ukrainian conflict when we can feel sure what the next day will bring,” she said.