M23 rebels in the east of the Democratic Republic of Congo are closing in on a second major city, Bukavu, as they continue their advance.
The militants’ progress comes despite international calls for a ceasefire and a resumption of peace talks.
Hundreds of thousands of people have been forced from their homes in recent weeks as a result of the rebel advance.
Last month, the Tutsi-led M23, which is backed by neighbouring Rwanda, seized Goma, the main city in the mineral-rich east. The Congolese government accuses Rwanda of sowing chaos in the region in order to benefit from its resources, a claim Rwanda has denied.
The M23 briefly captured Goma in a previous conflict in 2012, but the taking of Bukavu, which is the capital of South Kivu province, would represent a new phase in the turbulent recent history of the region.
The city, which borders Rwanda, is on the southern tip of Lake Kivu and is an important transit point for the local mineral trade.
Earlier on Friday, rebel fighters had entered the airport some 30km (19 miles) north of Bukavu, as the Congolese army and a militia allied with it withdrew without putting up much resistance.
But there were heavy clashes on the outskirts of Bukavu, South Kivu’s Deputy governor Jean Elekano, had told the BBC.
Residents contacted by the BBC said the authorities advised residents to remain indoors.
Speaking at the Munich Security Conference, DR Congo’s President Félix Tshisekedi called for Rwanda to be sanctioned accusing it of having “expansionist ambitions”, the AFP news agency reports.
“We will no longer put up with our strategic resources being plundered for the benefit of foreign interests under the complicit gaze of those who feed on chaos,” he is quoted as saying.
As well as being accused of backing the rebels, which Rwanda has denied, the country is also said to have its own troops in eastern DR Congo.
Rwandan President Paul Kagame has said his country’s priority is security. He has said Rwanda is in danger from Hutu rebels in DR Congo and has dismissed any threat of sanctions.
The news of the latest advance comes as the continent’s heads of state are due to meet at an African Union (AU) summit in Ethiopia on Saturday.