Mortar fire struck an apartment block in the rebel-held east Ukraine city of Donetsk overnight, yet another violation of a cease-fire between government forces and pro-Russian insurgents.

While both sides said Tuesday they saw progress on the ground in fulfilling an agreement to pull back heavy artillery weapons from the front-line, on Wednesday Kiev accused the rebels of violating a cease-fire imposed Sept. 5.

"The situation remains difficult," Col. Andriy Lysenko, spokesman for Ukraine's National Security and Defense Council, told journalists in Kiev on Wednesday. "Ukrainian detachments comply with the cease-fire regime but the terrorists continue provocations," he said, referring to the pro-Russian separatists.

Lysenko also said that eight servicemen had been wounded in fighting overnight, although none were killed.

A residential building in the north of Donetsk was heavily damaged by shelling, destroying at least two apartments. While RIA Novosti news agency quoted the rebels as saying that two people died in the attack, nobody at the scene could confirm any civilian casualties.

That part of the city has been the subject of almost daily shelling despite the cease-fire, as fighting centered around the government-held city airport nearby has caught many residential neighborhoods in the crossfire.

More than 3,500 people have been killed in the conflict since mid-April, the United Nations estimates, although the number of casualties is likely much higher. Much of the region's infrastructure and industry, from airports and bridges to coal mines, have been destroyed by shelling.

The continuing shelling comes after an agreement signed Saturday called for both sides to pull back heavy artillery, setting up a buffer zone that would allow for the cease-fire to be more effectively enforced. While withdrawals were underway in some villages in the region, the airport and other areas have remained hotspots.

The West and Ukraine have accused Moscow of propping up the rebels with arms and volunteers from the onset of the conflict. NATO has also reported Russian troops fighting on the Ukrainian territory.

Canadian Lt. Col. Jay Janzen, a NATO military spokesman, told The Associated Press on Wednesday that NATO has witnessed "a significant pullback of Russian conventional forces from inside Ukraine," but added that "some Russian troops still remain." He said it was difficult to say how many were still in Ukraine because they were moving across the border which is largely controlled by the separatists.