Rebel leaders claim a mutually-agreed prisoner swap with Ukraine will begin Saturday as a fragile ceasefire takes hold in the country's east, even amid accusations from both sides of early violations of the truce.

In the separatist stronghold of Donetsk, rebel leader Alexander Zakharchenko said that prisoners of war would be transferred to Kiev Saturday night and he hoped that Ukraine would uphold their side of the deal negotiated in Minsk on Friday.

"We hope that on Monday, the Ukrainian side will hold their transfer," Zakharchenko, whose official title is prime minister of the self-proclaimed "Donetsk People's Republic," told Russian news agency RIA Novosti.

Andriy Lysenko, Ukraine's National Security and Defense Council spokesman, told reporters the specifics of the exchange were still being worked out, but that Ukraine intended to execute the agreement "as fast as possible." Rebels are keeping more than 200 Ukrainians captive, Lysenko said.

The conditional ceasefire, which took hold at 6 PM local time Friday, follows four months of fighting between Russian-backed rebel forces and the beleaguered Ukrainian army.

The Organization for Security and Cooperation in Europe, an international watchdog, will oversee the deal, which was approved by envoys from Ukraine, the separatist leadership in Donetsk and Luhansk, and Russia.

Along with the release of all prisoners of war, the peace deal includes provisions for the withdrawal of heavy weaponry and channeling of humanitarian aid to war-ravaged cities in eastern Ukraine.

Though both sides have largely upheld their agreement to refrain from fighting so far, accusations of infractions have already been flung.

Lysenko said separatists fired at Ukrainian troops at least 10 times on Friday night, supposedly after the ceasefire was to take effect.

Zakharchenko also made claims to local media that the halt in hostilities was "not fully complied with," on the Ukrainian side, although he did not say when or where the alleged offenses occurred.

The Donetsk Mayor's Office released a statement on Saturday saying the morning had been quiet and the city had been operating normally, although an earlier release said the sound of shelling and gunfire was heard ringing through the city Friday evening.

Heavy bombardments destroyed several residential areas across the eastern Ukrainian city of Donetsk on September 4. Donetsk City Council reported that at least five people were killed in overnight shelling on the city, just 24 hours before a ceasefire took effect. Videos show the aftermath of an attack inside the apartment blocks on Kutuzov Street.

Many remain skeptical the peace agreement will hold in the rebel-controlled areas of eastern and southern Ukraine, where fighting has left at least 2,600 people dead and thousands more displaced, according figures released by the United Nations.

A host of unresolved issues remain at the heart of the conflict that has given rise to the worst tensions between Russia and the West since the end of the Cold War.

For now, Russian-speaking separatists will remain in control of their areas, but they ultimately still seek to split from Ukraine.

Both sides remain suspicious that the truce is being used as a guise to regroup and rearm before launching further attacks. The last ceasefire negotiated between the parties in June lasted just 10 days.

"The ceasefire is looking good for now but we know they (the Ukrainian side) are only using it to bring in more forces here and ammunition and then to hit us with renewed strength," one rebel commander identified by his nickname Montana told Reuters.

"Come what may, I would not trust Poroshenko. And it's not him making the call anyway but the Americans and that is even worse," he said.

Western leaders expressed similar caution during a two-day NATO conference held in Southern Wales, where the Ukraine conflict topped the agenda, along with the increasing threat from Islamic State extremists.

President Obama gave his closing speech at the final day of the NATO summit in Wales on September 5. Obama addressed the current unrest in Ukraine, calling on Russia to respect Ukraine's sovereignty and territorial integrity.

"With respect to the ceasefire agreement, obviously we are hopeful but based on past experience also skeptical that in fact the separatists will follow through and the Russians will stop violating Ukraine's sovereignty and territorial integrity. So it has to be tested," President Barak Obama said in closing remarks at the summit on Friday.

Hours before the ceasefire was announced, NATO allies had agreed to a rapid response force based in Eastern Europe that would mobilize quickly in the event of future Russian military aggression in the region.

The U.S. and European Union have also pledged further sanctions against Russia if they fail to pull back troops and heavy weaponry recently deployed to eastern Ukraine.

Moscow denies it has sent forces into Ukraine and said, "there will undoubtedly be a reaction from our side" if more sanctions are implemented.