Kinshasa, AP—Heavy fighting erupted on Wednesday as the Congolese army attacked one of the last remaining strongholds of the M23 rebels near the Ugandan border, forcing more than 10,000 Congolese to flee into Uganda, officials and humanitarian workers said.

Among those crossing the border on Wednesday was Bertrand Bisimwa, the civilian head of the M23 rebel movement, who was reported to be on his way to the Ugandan capital, Kampala, at the request of a mediator trying to bring an end to the 18-month rebellion, according to Uganda’s top military spokesman.

Bisimwa does not face arrest in Uganda, which has been mediating failed peace talks between the Congolese government troops M23 rebels since December, said Uganda Lt. Col. Paddy Ankunda. The talks stalled again earlier this month and within days clashes erupted between United Nations-backed Congolese forces and the rebels.

Another Ugandan army spokesman, Lt. Ninsiima Rwemijuma, said on Wednesday that two M23 rebels armed with AK-47 rifles surrendered to Ugandan border authorities on Wednesday morning. Rwemijuma, who speaks for Ugandan forces near the Congo border, said the militants were believed to be M23 fighters. They were disarmed and are now “under investigation,” he said.

Bunagana is believed to be the rebels’ last significant stronghold after a recent Congolese military offensive prompted the rebels to retreat—the reason many Congolese are now crossing into Uganda with some of their prized possessions.

Lucy Beck, a spokeswoman for the UN refugee agency in Uganda, said the Congolese crossing the border are now “too many to count.” Humanitarian workers on the Ugandan side of the border could hear gunfire from the Bunagana border post just a few kilometers (miles) away. The number of Congolese seeking refuge in Uganda escalated from 5,000 to more than 10,000 within hours on Wednesday, she said.

Julien Paluku, the governor of North Kivu province, confirmed that clashes were under way near Bunagana.

The M23 movement emerged in April 2012, the latest incarnation of an ethnic Tutsi rebel group dissatisfied with the Congolese government. Neighboring Rwanda, whose president is also an ethnic Tutsi, is widely believed to have provided weapons, recruits and training to the M23. Rwanda’s government vigorously denies the allegations, saying Congo’s government has failed to police its territory.

M23 briefly overtook Goma—a city of one million people—last November, but has been substantially weakened in the past year by internal divisions and waning Rwandan support, according to a United Nations group of experts.

The Congolese military has capitalized on these rebel setbacks by pushing ahead with new offensives beginning in August that have been supported by the most powerful UN force yet. After years of only protecting civilians, the UN is now actively aiding Congolese soldiers in pursuing their enemy.

In the past week, Congo has scored a series of successes and taken back half a dozen towns from rebel control to the cheers of local residents waving palm leaves and running alongside their vehicles.