Congolese rebels and allied Rwandan forces entered the outskirts of Goma on Sunday and the airport in the key eastern city was no longer in use, the top U.N. official in the country said, as thousands fled the fighting that also killed at least 13 peacekeepers.

Tens of thousands of people are fleeing towns in the Democratic Republic of Congo's east, as rebel forces close in on the city of Goma. Explosions and gunshots were heard in city - home to more than a million people - late on Sunday evening, according to local media reports.

Congolese rebels and Rwandan forces have closed Goma's airport, causing mass evacuations and raising concerns over humanitarian efforts.