European security watchdog OSCE said numerous anti-aircraft rounds were fired at one of its drones monitoring a shaky truce in eastern Ukraine, and the United States blamed pro-Russian rebels for Sunday's incident.

The unmanned aerial vehicle — deployed to help monitor the cease-fire between government forces and separatists — was not hit and it later landed safely, the Organization for Security and Cooperation in Europe (OSCE) said on Monday.

The incident occurred on the same day as the rebels elected a separatist leadership in eastern Ukraine in a vote that was denounced by Kiev and the West and further deepened a standoff with Russia over the future of the former Soviet state.