KINSHASA (Reuters) - Armed fighters led by the military commander of former Congolese rebel group M23 have crossed the border into the Democratic Republic of Congo from Uganda, Congolese officials said on Sunday.

The rebels had been in camps for demobilized fighters in Uganda following their defeat in 2013. Formerly, they were the largest of dozens of armed groups in the country and controlled huge swaths of the country’s mining heartland in the east.

Renewed violence would be a major challenge for President Joseph Kabila, who is trying to fend off mounting opposition over his decision to stay beyond his mandate which expired last month. Some observers fear tensions could spark a new civil war.

“They made an incursion yesterday from Uganda at Ishasha in two columns and the Congolese armed forces have dealt with them for now,” said government spokesman Lambert Mende, referring to a border crossing near Virunga National Park.

He said rebel commander Sultani Makenga was among them, leading one of the two columns.

Julien Paluku, governor of the North Kivu province, also confirmed the encroachment and condemned Uganda for allowing them to leave on U.N.-funded Radio Okapi.

In a brief telephone conversation with Reuters, he denied that there had been fighting. Officials in Uganda were not available for comment.

It was not clear where the fighters had gone. A letter from the ministry of defense sent to Reuters by a Congolese security official on Sunday requested an urgent investigation into allegations that 180 ex-M23 fighters had entered the country.

At its peak, M23 controlled North Kivu’s capital Goma but was driven out by U.N. and Congolese forces. Since then, the fighters have been scattered in camps in neighboring Uganda and Rwanda awaiting amnesties.

Many other armed groups remain active.

In the same province, armed fighters attempted to free prisoners from a facility in Beni overnight, according to a statement from local activist group The Centre of Study for the Promotion of Peace, Democracy and Human Rights (CEPADHO).

Reinforcements drove them away and one of the attackers was killed, the group said. It said the fighters were likely “Mai Mai” self-defense groups - militias originally created to resist Rwandan invasions.