The Rwanda-backed rebel group M23 is battling Congolese government forces in Congo's key city of Goma, causing the international airport there to be evacuated.

Nine United Nations peacekeepers in eastern Congo sustained injuries during fighting with the M23 armed group, the U.N. said, as the rebels close in on Goma.

A rebel alliance claimed the capture of the biggest city in the Democratic Republic of Congo’s mineral-rich eastern region this week, pushing back against resistance from government troops backed by regional and UN intervention forces.

The three peacekeepers — one from Uruguay and two from South Africa — were killed over the weekend in the fighting around Goma. More than 300 U.N. peacekeepers have been killed since the mission in Congo was deployed in 1999. The deaths have renewed questions about the future of the U.N. mission, known as MONUSCO.

The Rwanda-backed armed group M23 vowed on Thursday to march on the DR Congo capital, Kinshasa, as its fighters made further advances in the mineral-rich east of the country.