"I call on the M23 to stop its advance and to withdraw from Goma immediately," Simmonds said.

Oxfam, which is monitoring humanitarian conditions in the area, warned Thursday that the fall of Goma to the rebels poses "a very real risk of complete collapse of state authority and the humanitarian crisis reaching new depths."

It estimated that 120,000 people were in urgent need of help, with many sleeping in the open or in schools and other buildings without humanitarian aid.

Tens of thousands of Congolese, already displaced by previous rounds of fighting in the volatile region, have fled camps around the edges of Goma, according to UNICEF and the medical charity Doctors Without Borders.