Some 1,200 prisoners of war held by pro-Russian rebels in east Ukraine have so far been freed under the terms of a ceasefire deal agreed last week, Ukrainian President Petro Poroshenko announced on Monday.

"In the past four days we have managed to free 1,200 of our prisoners," Interfax-Ukraine quoted Poroshenko as saying as he visited the flashpoint southeastern port city of Mariupol.

The release was related to a prisoner swap that the warring parties had agreed under a truce deal hammered out Friday aimed at ending five months of bloody conflict in Ukraine's east.

A prisoner exchange was "task No. 1'' for the government, said Colonel Andriy Lysenko, a spokesman for Ukraine's National Security Council.

Poroshenko said upon his arrival in Mariupol that the insurgents had begun shelling checkpoints outside the city after learning that he was to visit the area for the first time since the fighting erupted in April.

"Mariupol is Ukraine. We will not surrender this land to anyone," he said on Twitter.

"They thought they would frighten me. But no one is afraid of them!" said Poroshenko, dressed in military fatigues for his visit to the only major city in the eastern conflict zone that is still under government control.

Seizing control of Mariupol, a major port and a heavy industry hub on the Sea of Azov, is apparently part of separatist plans to carve out a land corridor connecting mainland Russia to the Crimean peninsula, which Russia annexed in March.

A shaky peace

Addressing workers at a large metal plant in the embattled coastal city, Poroshenko's comments appeared to underscore that Kiev is unlikely to loosen what remains of its grip on the seditious east.

"Mariupol proved that we won't let anybody burn our city to the ground. This workers' Mariupol protected peace and calm in the city,'' Poroshenko told hundreds of factory workers wearing hard hats.

As Poroshenko addressed the crowd from a stage festooned with the blue and yellow of the Ukrainian flag, a shaky peace appeared to reign over much of east Ukraine. The city council of Donetsk said there had been no reported casualties overnight but that residents had reported hearing an explosion.

In Luhansk, another rebel-held eastern city that has seen some of the worst clashes, local authorities said there had been no fighting for the third night in a row.

Spokesman Lysenko told reporters that the rebels had violated the ceasefire half a dozen times, but that no Ukrainian serviceman had been killed in the past day and rebel forces had stopped using heavy artillery, limiting themselves to only mortar and rifle fire.

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

'Putin got what he wanted'

Despite the limited truce that began late on Friday there has been little progress toward a permanent political agreement that would end the stand-off in east Ukraine.

The deal came after Ukraine and the West said Russian troops had crossed into Ukraine, helping to turn the tide on months of government advances and forcing Kiev to talk to the rebels.

Weakened by military losses and the threat of a full-scale conflict with Moscow, Ukraine had to sit down with the self-declared leadership of Donetsk and Luhansk, therefore giving legitimacy to agents that Kiev had dismissed as "terrorists".

"[Russian President Vladimir] Putin got what he wanted, as Poroshenko had to talk to the Donetsk People's Republic and Lugansk People's Republic and recognise them as negotiating partners," said Stanislav Belkovsky, from the Moscow-based National Strategy Institute.

Political analyst Maria Lipman agreed that the truce deal was a victory for Putin. "This was Putin's aim from the beginning: To have leverage to be able to dictate his conditions to Ukraine, not only for the situation in the east but overall, politically."

(FRANCE 24 with AFP, AP and REUTERS)