GOMA, Congo — Rebels believed to be backed by Rwanda fired mortars and machine guns Monday on the outskirts of the provincial capital of Goma, threatening to capture one of the largest cities in eastern Congo in a development that could drag this Central African nation back into war.

The gunfire and explosions erupted in the early afternoon, with shells landing as far away as the international airport and near a United Nations position, causing flights to be rerouted and prompting the United Nations to evacuate most of its employees, according to UN officials.