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Sergei Grits, Associated Press
Pro-Russian rebels drive an armored truck in Donetsk, eastern Ukraine, Sunday, Sept. 7, 2014. Strong explosions were heard early Sunday on the outskirts of the main rebel-held city in eastern Ukraine near the airport, raising new fears that a cease-fire signed two days ago is on the verge of collapse. Blasts powerful enough to be heard in downtown Donetsk came from the area near the airport, which has been under the control of government troops since May and has come under unremitting attacks from pro-Russia separatist rebels since then.

KIEV, Ukraine — A cease-fire between pro-Russian rebels and Ukrainian government troops appeared to be largely holding Monday in eastern Ukraine, a day after shelling and clashes rattled nerves and imperiled the peace deal.

In a surprise announcement, Ukrainian president Petro Poroshenko was traveling Monday to the embattled coastal city of Mariupol, the regional administration of Donetsk said. That trip would be a symbolic show of strength in a strategic government-held area that has come under rebel fire in recent days, and proves Kiev is unlikely to willingly loosen what remains of its grip over the rebellious east.

The city council of Donetsk said there had been no reported casualties overnight and it expected public transport to be running on Tuesday. No shelling or explosions were heard overnight in downtown Donetsk.

Col. Andriy Lysenko, a spokesman for Ukraine's National Security Council, told reporters Monday there had been about half a dozen rebel violations of the cease-fire. But he said no servicemen had been killed in the past day and rebel forces had stopped using heavy artillery, only mortar and rifle fire.

"That's a big achievement," he said. "We understand that the cease-fire imposes some discipline to our enemies and also allows our military to bring its reserves closer."

A successful cease-fire would be a landmark achievement for all sides in a conflict that has dragged on for nearly five months and claimed at least 3,000 lives, according to a U.N. estimate issued Monday.

But despite the cease-fire, there has been little agreement between Kiev and the separatist rebels on a political settlement that would permanently end the standoff in eastern Ukraine.

The southeast has been a key hotspot in the last two weeks. The area around Mariupol had remained relatively untouched by violence until then, when rebel forces pushed toward the city, shelling the city's outskirts as recently as Saturday. The port is strategically located on the Sea of Azov coast, raising fears that if it fell, Moscow-backed rebels could link up mainland Russia with Crimea, the Black Sea peninsula annexed by Russia in March.

In other developments, 15 Ukrainian soldiers were released by rebel forces, the Interfax-Ukraine news agency reported Monday. Lysenko said a prisoner exchange with the rebels was "task No. 1" for the government during the cease-fire.

The cease-fire was imposed late Friday, but was thrown into peril over the weekend by the shelling of Mariupol and fighting near the airport of the rebel-held city of Donetsk. A previous 10-day cease-fire in June was riddled by reports of violations on both sides.

Peter Leonard contributed to this story from Donetsk, Ukraine.