Fighting erupted on Tuesday in eastern Democratic Republic of Congo, three days after a call by African leaders for a ceasefire and a brief lull in the conflict.
M23 fighters attacked Congolese army positions in South Kivu province at dawn, local and security sources told AFP.
The resurgence comes after east and southern African leaders called on their general staff to propose a plan for implementing an "unconditional" ceasefire by Thursday, in a conflict which has killed thousands and driven vast numbers from their homes.
The M23 has in recent months swiftly seized tracts of territory in mineral-rich eastern DRC after again taking up arms in late 2021, in a country plagued by numerous conflicts for decades.
'Detonations of heavy weapons'
The armed group began advancing in South Kivu after taking control of Goma, the capital of neighbouring North Kivu province that borders Rwanda, at the end of last month.
Clashes took place on Tuesday near the village of Ihusi, around 70 kilometres (43 miles) from the provincial capital Bukavu and 40 kilometres from the province's airport, according to security sources.
Several local sources reported "detonations of heavy weapons".
Kavumu airport is used by the Congolese army to transport reinforcements of men and equipment to the region and its main military base is located nearby.
Preparing for M23 offensive
Bukavu has been preparing for an M23 offensive for several days, with schools shuttering in the city on Friday as residents began to flee and shops closed over fears of an imminent attack.
Banks were still shut in the city on Tuesday.
The capture of Bukavu would give full control of Lake Kivu to M23.
The anti-governmental group, which claims to want to "liberate all of DR Congo" and oust President Felix Tshisekedi, has attempted in recent days to advance into the highlands overlooking the main road to Bukavu to cut off the DRC army's supply lines.
Burundian soldiers
But Burundian soldiers, who are in eastern DRC to support the Congolese army, stopped the M23 advances, security sources said.
Around 10,000 Burundian soldiers are deployed in South Kivu, according to a security source.
Bujumbura sent at least one additional army battalion to the area on Friday, a security source told AFP.
The M23 has begun setting up its own administration in Goma, a city of one million people, launching recruitment campaigns, including to create a police force.
Humanitarian situation
The humanitarian situation in Goma is worsening with no running water in large parts of the city and residents forced to take water from Lake Kivu, where bodies were recovered after the fighting.
An increase in cholera cases has been seen in the region, particularly among people displaced by the conflict, UN humanitarian agency, OCHA, said.
The crisis in eastern DRC is set to be discussed at an African Union meeting in Addis Ababa on Friday.
With the recent intensification of the conflict, calls for de-escalation from the international community have increased amid fears the fighting could lead to a regional war.
Diplomatic efforts
But diplomatic efforts to resolve the conflict that has lasted for more than three years have so far been unsuccessful.
The DRC has called for "targeted sanctions" against Rwanda but with little effect.
Kinshasa accuses Kigali of wanting to plunder natural resources in the DRC, such as tantalum and tin used in batteries and electronic equipment, as well as gold.
Rwanda denies this, saying it wants to remove armed groups it believes pose a permanent threat to its security, notably the Democratic Forces for the Liberation of Rwanda (FDLR), created by former Hutu leaders of the 1994 genocide against Tutsis in Rwanda.
Different conflicts and rebellions have plagued the country for more than 30 years.
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