Rwandan-backed rebels who have swept through eastern Congo this week met resistance from the army and its allies including Burundian troops as they tried to push south on Friday, a Congolese official and another source said.
Rwanda denies sending troops into neighbouring Democratic Republic of Congo, where UN investigators said in December thousands of Rwandan soldiers had been deployed in support of the M23 rebel group.
The rebels took control of the city of Goma in the volatile, mineral-rich east on Tuesday.
M23's latest offensive along the western shores of Lake Kivu has been checked in the last couple of days, South Kivu's governor Jean Jacques Purusi said.
Road to Bukavu
A force of around 1,500 comprising the army, local militias and Burundians has deployed to defend the town of Nyabibwe on the road to Bukavu, a person with direct knowledge of the situation on the ground said, declining to be identified for security reasons.
Relations between Rwanda and Burundi are already hostile, and fighting between their two armies in Congo would heighten the risk, already expressed by the United Nations, that the conflict could spill into a regional war.
M23 spokesman Willy Nangaa said the rebels have been fighting Burundian troops for the last few weeks in the defence of Goma and other cities.
An African diplomat said the situation was alarming.
"The risk of regionalization of the conflict is real. The Rwandan, Congolese and Burundian armies are already clashing on the ground in eastern Congo. The capture of Goma was a wake-up call for the international community, which realizes the danger now," the diplomat said.
Request for support
Burundi's military has declined to comment on this week's developments in Congo, where it is present on a request from the government.
A Burundian official said his government received increasing requests from the Congolese authorities over the past two years to support the Congolese army.
"Our country has also paid a heavy price, which is why we have asked our two neighbours to sign a ceasefire and negotiate," the official said.
Authorities in Bukavu sought to recruit civilian volunteers to defend the city, the armed group's likely next target.
"If this is how they want war, then may God forgive us, but we will fight ... to defend our own," Albert Kahasha, a former rebel and ex-army officer, told a rally to recruit volunteer fighters on Thursday.
Rebel advance
Fighting around the town of Mukwinja, around 70km (43 miles) north of Bukavu, a city of over one million people, was slowing the rebel advance early on Friday, sources said.
UN sources estimate there are several thousand Burundian troops in eastern Congo, where they are deployed in support of the Congolese army. On Sunday, Rwanda told the U.N. Security Council that there were 10,000 Burundian troops there.
The absence of U.N. peacekeepers in South Kivu heightens the risk of an escalation in fighting, the UN said on Thursday, adding that there were reports of Rwandan forces crossing into Congo in the direction of Bukavu. Rwanda did not immediately respond to a request for comment.
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