DR Congo rebels have seized the airport in Goma, the main city in the country's mineral-rich east after several days of fighting, a United Nations official told reporters Tuesday.

"The airport is under the control of the M23," the official said on condition of anonymity, referring to the rebel group.

Fresh fighting broke out Monday on the outskirts of Goma with witnesses reporting weapons fire to the north and northwest of the city causing residents to flee south or towards the Rwanda border.

Kinshasa had dismissed a rebel demand for direct talks as "irrational rantings" and said the M23 rebels were "fictitious forces put in place by Rwanda to hide its criminal activities in DR Congo."

"We prefer to negotiate with Rwanda, the real aggressor," spokesman Lambert Mende told AFP.

The United Nations has also charged that the rebels are backed by Rwanda as well as Uganda, charges both countries deny.

UN peacekeeping chief Herve Ladsous said the United Nations could not confirm Rwandan backing but said M23's "attacking forces are well-equipped and very well-supplied".