The U.N. refugee agency voiced concern at the "rapidly deteriorating" situation in the eastern Democratic Republic of Congo on Friday, saying the war had left around 350,000 displaced people with no roof over their heads.

Rwandan-backed M23 rebels seized eastern Congo's largest city of Goma last month and have been inching south in an advance that a local official has said may trigger a broader catastrophe in an area already housing thousands of displaced people.

UNHCR spokesperson Eujin Byun told journalists in Geneva by video link that around 350,000 internally displaced people had no shelter as 70,000 emergency shelters had been destroyed around Goma and Minova. Unexploded munitions had also made it unsafe for them to go home.

About 70% of Goma's camps are destroyed, and others in Minova are damaged, according to UNHCR.

"Hundreds of thousands of people are now living in makeshift accommodation, including churches and hospitals," Byun added.

The agency also reported an increase in crime and said the danger of disease was growing as it and other agencies struggle to provide aid amid the fighting.

More than 80 civilians were killed in a late-night attack by the Codeco militia on a cluster of villages in eastern DRC this week, the UN peacekeeping mission Monusco said on Thursday.

Codeco, one of many militias fighting over land and resources, has frequently attacked the displacement camps, which have mushroomed since M23 began its advance.

Rwanda has been accused by DRC, the UN and Western countries of supporting M23 with its own troops and weapons, a charge Kigali denies.

At least 3,000 people have been killed and hundreds of thousands displaced by the recent fighting.