Eight UN peacekeepers - six Pakistanis, a Russian and a Serb - were killed Tuesday when a Puma helicopter crashed in the troubled eastern Democratic Republic of Congo, UN and Pakistani officials said.

"While undertaking a reconnaissance mission in Congo, 1 PUMA Helicopter crashed. Exact cause of crash is yet to be ascertained," the Pakistani military's media wing said.

A spokesperson for UN chief Antonio Guterres in New York confirmed the crash and gave the nationality of all eight victims.

Pakistani Prime Minister Imran Khan expressed his "deep sense of shock and grief,” paying tribute to the global peace effort by the country's armed forces.

Congolese military authorities in North Kivu said M23 rebels had "shot down" the aircraft. But the group denied this, instead claiming the Congolese military was responsible for the crash.