Rebels in the Democratic Republic of Congo have seized a military airport in Goma, the main city in the country's mineral-rich east, after several days of fighting with government forces backed by a UN peacekeeping force, a United Nations official has told reporters.

"The airport is under the control of the M23 (Rebels)," the official said on condition of anonymity on Tuesday.

The advancement came after rebels fired mortars on the outskirts of Goma, the provincial capital, threatening to capture one the largest cities in the country's eastern region. 

Shells landed near the international airport and a United Nations position, prompting the UN to evacuate most of its employees, according to UN officials. 

The violence erupted just hours after the M23 rebels said they were halting fighting to negotiate with the government.

But Lambert Mende, a government spokesman, told the Associated Press news agency by phone that negotiations are out of the question, saying DR Congo will not give in to the "blackmail" of a Rwandan-backed group.

"We refuse to enter into negotiations with M23. Because it's Rwanda, not the M23, that is responsible," said Mende.

"If Goma falls, it's going to create a whole other set of problems. We refuse systematically to speak to them (M23). Because if we do, it would be a way to wash away Rwanda's responsibility."

DR Congo and Rwanda have already fought two wars, the most recent of which ended in 2003 after lasting nearly six years.

On Monday, both nations accused the other of firing mortars across the narrow border which runs on one side of Goma, a city of one million that is the economic heart of Congo's mineral rich region.

Source: Agencies