Russia continues to supply weapons to the uncontrolled by Ukrainian government territory of Donbas, so to de-escalate the situation in eastern Ukraine the most important issue is to control the uncontrollable part of the Russian-Ukrainian border, the head of the Ukrainian delegation at the negotiations within the OSCE Forum, Ihor Prokopchuk, has said.

"The uncontrolled border still requires improvements for de-escalation, as Russia continues to supply its weapons to Donbas," the Ukrainian OSCE mission quotes Prokopchuk as saying on his social media Twitter microblog on Thursday.

According to him, militants continue to fire from residential areas at positions of ATO forces on the territories under their control. "We condemn the cynical practice of Russian separatist forces firing from residential areas, as Special Monitoring Mission (SMM) of the OSCE reports," Prokopchuk said.

Prokopchuk added that during May, 28 Ukrainian soldiers were killed, 96 soldiers wounded because of Russia's failure to comply with its obligations. Besides, civilians living in the conflict zone suffer because of militants' actions.

According to him, a stable cease-fire in eastern Ukraine remains a key requirement in the implementation of the Minsk agreements. The head of the Ukrainian delegation said that Russia "deprives Donbas from establishing peace" interpreting the Minsk agreements in its own manner and putting forward their conditions for the suspension of fire attacks.

As reported in February, President of Ukraine Petro Poroshenko signed a decree providing for a delegation from Ukraine to participate in negotiations within the framework of the OSCE Forum on cooperation in the security sphere, the Joint Consultative Group and the Advisory Commission on open skies. He also appointed Permanent Representative of Ukraine to International Organizations in Vienna Ihor Prokopchuk as its head.