The OSCE Special Monitoring Mission to Ukraine (OSCE) observed an explosion 300 to 500 meters from its location in the Zolote disengagement area in Luhansk region of Ukraine on Sunday.

"At 11:04 on 20 May, two SMM patrols consisting of ten members and four armored vehicles positioned themselves about 20m south of a checkpoint of the armed formations inside the disengagement area near Zolote (government-controlled, 60km west of Luhansk) on its southern edge," the mission said in a statement.

"All patrol members were outside the vehicles, five of whom were speaking with two unarmed members of the armed formations, and saw an additional five to eight members of the armed formations (one armed with an assault rifle (AK-type), the remainder unarmed) walking around the checkpoint inside the disengagement area," it said.

"At 11:06, the patrol members heard an undetermined explosion 300-500m north (assessed as inside the disengagement area). (The SMM had previously observed an anti-aircraft gun (ZU-23, 23mm) about 10m north-west of the abovementioned checkpoint inside the disengagement area," the monitors said.

"Both patrols immediately left the area and returned safely to their bases in Severodonetsk (government-controlled, 74km north-west of Luhansk) and Kadyivka (formerly Stakhanov, non-government-controlled, 50km west of Luhansk), respectively," the mission said.