Lava from an erupting volcano in the DR Congo was still flowing Friday, a vulcanologist said, threatening wildlife in a national park that is home to some of the world's last mountain gorillas.

The amount of lava being spewed from Mount Nyamulagira was "sustained and won't stop as soon as all that," Goma Vulcanological Observatory director Karume Katcho told AFP six days after the eruption started.

The lava trail initially gained speed as it hit steep ground in the Virunga National Park, he said, then slowed down by Friday to be about seven kilometres (four and a half miles) long and 200 metres (655 feet) wide.

For the first time since the eruption, "the wind changed last (Thursday) night and volcanic ash fell on the town of Goma," more than 20 kilometres from the volcano, without causing any damage, Katcho said.

Even in Sake, a town in the path of the lava flow about 20 kilometres south of Nyamulagira, the population is not directly threatened, in spite of the ash that has rained down.

"People are on the alert and are taking hygienic precautions. Tap water isn't affected but rain water will be highly contaminated," Katcho said. At the beginning of the week, relief agencies announced that they would supply fresh water in tanks.

Katcho estimated that the lava had burnt about 11 hectares (27 acres) of the Virunga park, which contains large numbers of species, including elephants, hippopotamus and buffalo, along with a range of reptiles and birds.

The worst threat to the park in recent years has come from human beings, in the shape of rebel bands and government troops, who kill wildlife, including endangered species.

The park lies in the eastern part of the Democratic Republic of Congo that is still most ridden by conflict between the Congolese army (FARDC), renegade soldiers and rebels from neighbouring Rwanda and Uganda.

A UNESCO World Heritage Site, the park is home to around 200 of the world's last 750 mountain gorillas but they are east from the volcano, near the border with Rwanda.

Nyamulagira is a twin to Mount Nyaragongo, which lies about 12 kilometres southeast. The two are the only active volcanos among the eight forming the Virunga chain on the borders of the DR Congo, Uganda and Rwanda.

In January 2002, Nyaragongo erupted spectacularly, with a lava flow that reached Goma, destroyed about 80 percent of the town, and killed 147 people.

The last two eruptions of Nyamulagira, considered the most active volcano in Africa, were in May 2004 and November 2006. No human settlements were affected. Eruptions can last from several days to several months, like that of September 1991, which ended at the beginning of 1993.