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.

The calls come after 13 soldiers serving with peacekeeping forces in the DR Congo were killed in clashes with M23 rebels, UN and army officials have said. South Africa said nine of its soldiers were killed while deterring an advance on Goma.

Congolese M23 rebels entered the centre of the eastern city of Goma on Monday, two witnesses said, hours after they claimed to have seized the city despite the United Nations Security Council demanding an end to the offensive.