United Nations Secretary-General Antonio Guterres has warned that eastern Congo's M23 conflict risks spiralling into a broader regional war, his spokesperson said in a statement, condemning the rebels' capture of another strategic town, Sake.

Fighting has flared more fiercely in Democratic Republic of Congo's mineral-rich east since the start of the year as the Tutsi-led M23 group seized control of more territory than ever before, prompting thousands more people to flee their homes.

Congo and the United Nations accuse neighbouring Rwanda of fuelling the three-year M23 insurgency with its own troops and weapons. Rwanda denies this.

"This offensive has a devastating toll on the civilian population and heightened the risk of a broader regional war," UN spokesperson Stephane Dujarric said on Thursday.

The Secretary-General "calls on all actors to respect the sovereignty and territorial integrity of the Democratic Republic of the Congo and to put an end to all forms of support to armed groups," Dujarric said.

After seizing the town of Minova on Tuesday, M23 fighters have continued their advance, moving into the town of Sake, around 20 km from the provincial capital Goma, the largest city in eastern Congo.

The national army spokesman did not respond to requests for comment about the situation in Sake on Thursday, but the UN statement condemned "the recent seizure of Sake, which increases the threat to the town of Goma".

The M23 briefly managed to take over Goma during a previous rebellion in 2012, prompting international donors to cut aid to Rwanda. Even then, the rebels did not hold as much ground as they do now.