Around 400 000 people need support or protection after fleeing a volcano eruption and a wave of aftershocks in eastern Democratic Republic of Congo the United Nations said on Friday.

At least 31 people died when Mount Nyiragongo sent a wall of lava spreading towards the city of Goma on Saturday, destroying 3 000 homes along the way and cutting a major road used to bring aid to the strife-torn region.

The lava stopped just short of the city limits, but thousands more people fled early on Thursday when the government warned that the volcano, one of the world's most active, could erupt again.

Many escaped to Sake, a town 20 km northwest of Goma that is prone to cholera outbreaks, the United Nations children's fund (UNICEF) said.

"With an increased risk of a cholera outbreak, we are appealing for urgent international assistance to avert what could be a catastrophe for children," UNICEF's representative in Congo, Edouard Beigbeder, said.

Authorities issued an evacuation order around 1 am local time on Thursday after radar images showed molten rock flowing under Goma.

The movement of magma caused cracks in the surface and hundreds of earthquakes, which could allow it to burst through to the surface in a fresh eruption, the Goma Volcano Observatory (OVG) said.

The frequency and intensity of the ground tremors had lessened in the last 24 hours, suggesting the risk of a fresh eruption was subsiding, Celestin Kasareka Mahinda of the OVG said on Friday.

"I don't think we will have a second eruption. The problem is the risk of fractures, but the risk is small, around 20%," he told Reuters.

Authorities have reopened the main road which was split in two by the lava on Saturday, the UN Office for Coordination of Humanitarian Affairs (OCHA) said on Thursday.

Goma is major humanitarian hub supplying aid to a region hit by decades of unrest.