The M23 has made rapid advances through eastern DR Congo in recent weeks
M23 rebels in the east of the Democratic Republic of Congo have captured the airport serving the key city of Bukavu as they continue their advance.
The militants' development comes despite international calls for a ceasefire and a resumption of peace talks.
Hundreds of thousands of people have been displaced in recent weeks as a result of the rebel advance.
Last month, the M23, which is backed by neighboring Rwanda, seized Goma, the main city in the mineral-rich east.
In the fighting for the airport, which is in the town of Kavumu, the Congolese army and its allied militia withdrew without putting up much of a fight.
The rebels are now some 30km (19 miles) from Bukavu, which is the capital of South Kivu province. Its deputy governor, Jean Elekano, has told the BBC that there was heavy fighting between the army and the M23 on the outskirts of the city.
He said the army was still in control and fighting back.
Bukavu residents contacted by the BBC said there was growing panic and fear. The authorities have advised residents to remain indoors.
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.
The head of the AU commission, Moussa Faki Mahamat, is quoted by the AFP news agency as saying that a ceasefire "must be observed" adding that "military campaigns are not going to solve these problems. There is a general mobilization of Africa today on this issue, and I hope that we will be able to impose this ceasefire."
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 Congolese government has accused Rwanda of illegally annexing a large slice of the mineral-rich territory.
