Democratic Republic of Congo's army clashed with fighters from the M23 rebel group close to the eastern city of Goma on Sunday, in what appeared to be the most serious combat for several weeks.
In a statement, M23 said the fighting started at 2 p.m. local time (1200 GMT) when the army attacked its positions in the town of Mutaho, some 7 km (4 miles) from Goma, a city of a million inhabitants.
The rebel group, which sparked international outcry when it seized Goma in November, said its troops were responding firmly.
"Kinshasa's army has just launched a large-scale attack on M23's positions at Mutaho," Vianney Kazarama, an M23 spokesman, told Reuters.
Colonel Olivier Hamuli, an army spokesman, said the rebels had attacked government forces.
"The initiative for the fighting came from M23. It was them who attacked us at Mutaho," he said by telephone from Goma.
The M23 has accused the army and pro-government militia of trying to goad its troops into combat for several weeks.
The fighting comes as the deployment is under way of a new 3,000-strong U.N. Intervention Brigade with a tough mandate to take the fight to armed groups in eastern Congo.
The force - which will be made up of troops from South Africa, Tanzania and Malawi - has begun patrolling and is approaching full strength.
U.N. peacekeeping chief Herve Ladsous told Reuters on Sunday the 17,000-strong U.N. force in Congo would begin using unarmed drones on a trial basis to monitor the war-torn region.
U.N. peacekeeping troops have been in eastern Congo for more than a decade but the complex conflict has dragged on, killing millions through violence, famine and disease since the 1990s.
Peace talks between the Congolese government and M23 in Kampala, the capital of neighbouring Uganda, have stalled.
Some 66,000 Congolese refugees have fled into Uganda since the Ugandan rebel Allied Democratic Forces (ADF), an Islamic group, and elements of the Somalian al Qaeda-linked Shabaab movement struck the town of Kamangu on Thursday.
On Sunday, local officials said that ADF and Shabaab fighters had captured the town of Kikingi, close to the Ugandan border, when police fled after a short firefight.
"They are looting and robbing, as they did at Kamango," said Amisi Kalonda, the regional administrator. "The army is stationed not far from there and is going to dislodge them."
The ADF waged an insurgency against the Ugandan state in the late 1990s from its bases in Uganda's Ruwenzori Mountains and across the frontier in the eastern Congo jungle.
A Ugandan government offensive that ended in 2001 quelled the uprising and pushed its remnants deeper into eastern Congo. The group had since kept a low profile.
Uganda is worried that a buildup in the capacity of the ADF could pose a threat to its Lake Albert region, where crude reserves estimated at 3.5 billion barrels have been discovered.
Al Shabaab claimed responsibility for a 2010 bombing in which 79 people were killed while watching the soccer World Cup final in revenge for Uganda's deployment of troops in Somalia.