Ukrainian forces killed 12 pro-Russian separatists in clashes around the government-controlled airport in Donetsk Friday, as the cease-fire agreement signed by the two sides Sept. 5 continues to falter, Reuters reported.

The deal between Ukraine and the pro-Russian rebels was subsequently refined to include an 18.6-mile-wide buffer zone, but recent fighting there has prevented an international commission representing Ukraine, Russia and the Organization for Security and Cooperation in Europe, or OSCE, from working in the proposed zone in eastern Ukraine, Bloomberg News reported. A recent bomb blast near the Red Cross headquarters in Donetsk killed one worker.

Ukrainian military representative Andriy Lysenko discussed the attack on the Donetsk airport with reporters Saturday, saying two Ukrainian troops were killed, as well as the 12 pro-Russian separatists. He did not provide details about their deaths. “The airport of Donetsk remains the priority target for terrorists. Yesterday they resorted to a few, fortunately unsuccessful, attempts to storm it,” Lysenko said. Ukraine once again accused Russia of providing support to the separatists, a claim Moscow has denied in the past.

The United Nations expressed its concern over recent cease-fire violations because they prevent aid and other resources from being delivered to residents of eastern Ukraine.

“Serious cease-fire violations are reported daily and shelling has intensified in parts of Donetsk and Luhansk regions. As a result, access is impeded, displacement increased, and civilians in need of assistance and support endangered. In addition, in-country humanitarian agencies envisage that this deterioration of the humanitarian situation will be further compounded by the rapidly approaching winter with many [internally displaced persons] housed in unwinterized shelter,” the U.N Office for the Coordination of Humanitarian Affairs said in a report published Friday.

The U.N. report said 3,627 people, including the victims aboard Malaysia Airlines Flight 17, aka MH17, have been killed during the recent conflict in Ukraine. It said the fighting has led 426,995 people to flee the country.