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Story highlights

NEW: The sound of shelling breaks the silence near Debaltseve

A peace agreement was signed this month in Minsk, Belarus

It's unclear how it will be implemented and whether it will stick

(CNN) —  

Ukraine President Petro Poroshenko declared a ceasefire went into effect at midnight Saturday local time (5 p.m. ET) between the Ukrainian military and pro-Russian militants in the nation’s east.

President Petro Poroshenko ordered the Ukrainian military to hold its fire against pro-Russian militants in the nation’s East. The separatist Donetsk People’s Republic also announced a truce.

The ceasefire aims to end 10 months of violence in eastern Ukraine.

But an apparent violation was reported less than 90 minutes after the ceasefire began when Ukraine’s Anti-Terrorist Operation Command reported that a military post near Zolote in the Luhansk region has been shelled by mortars.

As a positive sign, CNN reporters in Ukraine reported the shelling quieted shortly after midnight.

Ceasefire holding?

The “ceasefire is holding,” according to chief monitor Ertugrul Apakan of the Organization for Security and Co-operation in Europe.

Referring to the first 12 hours of the ceasefire, Apakan said the ceasefire was being respected overall, with some exceptions.

Apakan noted violations in the town of Debaltseve, where an exchange of artillery fire began after the ceasefire was implemented.

Ceasefire monitors said they were blocked from traveling into the town. Deathly silence in the area was broken Sunday by the sound of shelling and at times civilians still fleeing for their lives, CNN’s Nick Paton Walsh reported.

In the area around the city of Donetsk, there were reports of a mortar round, three uses of a multiple-launch rocket system and small-arms fire in the first hour of the ceasefire, but the “situation was calm,” the chief monitor said.

There were also violations reported before dawn in the city of Luhansk, he said.

The OSCE mission “calls on all parties to fully adhere to the ceasefire in all locations,” Apakan said.

Ukraine’s Defense Ministry concurred that combat activity by “illegal armed groups” fell after the ceasefire went into effect, but said the situation in Donetsk and Luhansk regions remain “extremely difficult.”

In a statement, defense spokesman Andriy Lysenko said that since the beginning of the ceasefire, “enemy forces” initiated fire from artillery, grenade launchers, mortars and small arms against Ukrainian military positions in the area near the towns of Debaltseve and Shyrokine.

Ukrainian forces had suffered no casualties since the announcement of the ceasefire, said Lysenko.

Doubts, mistrust

The peace agreement signed in Minsk, Belarus, comes with questions over how it will be implemented and whether it will stick. Mistrust is high on both sides, adding to the doubts.

The town of Debaltseve, a strategically located railroad hub in the east, is shaping up as a crucial piece of the ceasefire.

In his speech announcing the ceasefire, Poroshenko said Ukrainian troops control the town and warned militants to stay away.

“I think the fact of using the tense situation on the outskirts of Debaltseve by militants-terrorists-mercenaries poses a great threat, which can violate the ceasefire regime,” Poroshenko said. “I am warning all participants of the Minsk negotiations, including the Russian Federation, which identifies itself as the guarantor of the reached agreements.”

CNN’s Nick Paton Walsh, reporting from the eastern town of Donetsk, said separatists claim to have encircled Debaltseve.

Poroshenko has warned that if the separatists do not abide by the ceasefire, he will impose martial law throughout Ukraine’s territory, his spokesman Andrey Zhigulin told CNN.

“The Ukrainian President has emphasized the need for monitoring by [Organization for Security and Cooperation in Europe] to confirm the ceasefire,” a statement from Poroshenko’s office said. “Petro Poroshenko and Barack Obama have agreed on further coordination of efforts in case of escalation of the conflict.”

The White House said in a statement, “The President spoke with Ukrainian President Poroshenko today to express his sympathy for the mounting toll of the conflict in eastern Ukraine and his deep concern about the ongoing violence, particularly in and around Debaltseve.”

Quiet after ferocious shelling

Shelling rocked two eastern Ukrainian cities Saturday ahead of the ceasefire, raising fears that the deal to end a bitter 10-month conflict may be in jeopardy.

Both incoming and outgoing artillery could be seen in the vicinity of the coastal city of Mariupol, and there was significant shelling in Donetsk city, CNN teams reported.

Ukraine’s National Defense and Security Council reported seven soldiers killed and 23 wounded in 24 hours. The council said additional forces were crossing the border from Russia into Ukraine in the area of Novoazovsk.

Igor Martinov, separatist-appointed mayor of Donetsk, said two people were killed and four injured by shelling in the city center Saturday afternoon. Photographs of the scene appeared to show one of the dead in camouflage gear and the other in civilian clothing.

OSCE urges reduction of hostilities

Both sides are expected to start pulling back their heavy weapons from the front lines Monday, creating a buffer zone at least 50 kilometers (31 miles) wide.

The Organization for Security and Co-operation in Europe, which already has a monitoring mission in Ukraine, has been given the challenging task of overseeing the process.

It also envisions prisoner swaps, new local elections in the conflict-affected regions of Luhansk and Donetsk, and a law granting amnesty to those involved in the the fighting.

However, Svyatoslav Tsegolko, a spokesman for the Ukrainian president, said that those responsible for shooting down Malaysia Airlines Flight 17 over Ukraine last summer will not be given an amnesty under the Minsk agreement.

This could be a potential hurdle to keeping the separatists on board.

Western governments believe rebels used a Russian-supplied missile system to shoot down the jet, having mistaken it for a military aircraft, while Kiev says it was a Russian officer who pressed the button. The separatists and Russia have denied any responsibility.

Russia ‘not a participant’ in conflict

The Kremlin has said it hopes to see the latest peace plan succeed.

Russia has steadfastly denied accusations by Kiev and the West that it is sending forces and heavy weapons into Ukraine.

But Kiev has accused it of continuing to do so even since the peace plan was signed in Minsk.

A previous peace agreement in September swiftly disintegrated.

A defense spokeswoman said Britain was not providing lethal aid to Ukraine. The trucks in question can offer protection to military personnel on the move but are not close combat vehicles, she added.

CNN’s Frederik Pleitgen reported from Mariupol, Ukraine, and Faith Karimi wrote from Atlanta. CNN’s Laura Smith-Spark and Nick Paton Walsh contributed to this report.