Rebels release captive Ukrainian troops

• 73 Ukrainian troops are released by pro-Russian rebels in Donetsk


• President Petro Poroshenko talks about ceasefire violations with German leader


• As shelling continues, officials in Donetsk describe the situation as "critical"

(CNN) -- Pro-Russian rebels released dozens of captive Ukrainian troops Sunday as part of a ceasefire deal.

Ukrainian President Petro Poroshenko said in a Twitter post that 73 troops were released in Donetsk.

But despite the ceasefire, violence continued to flare in the volatile region. As shelling rocked the city throughout the day Sunday, local officials described the situation as "critical."

Poroshenko spoke with German Chancellor Angela Merkel over concerns that the ceasefire deal -- brokered earlier this month -- is being violated. They "agreed to make further efforts to settle the situation peacefully," Poroshenko's office said in a statement.

In an interview with TV Tsentr pm Saturday, Russian Foreign Minister Sergey Lavrov said the truce agreed upon in Minsk, Belarus, appeared to be holding generally and that Moscow, at least, is ready to work toward a long-term peace.

"Sporadic exchanges of fire occur on both sides, but the process of establishing (a) durable peace is still in progress," he said, according to parts of that interview published by the state-run Itar-Tass news agency.