Lava from Mount Nyamulagira, in a sparsely populated area of the Democratic Republic of Congo, is threatening about 40 endangered chimpanzees.
Mount Nyamulagira is located around 25km (16 miles) from the eastern city of Goma, BBC News
reports.
The lava from the volcano is flowing into the surrounding Virunga National Park.
Emmanuel de Merode is the Park director and he informed the BBC that staff were working with the civilian and military authorities. They are determining the best course of action to follow.
The lava flow is being monitored and hourly reports are expected.
Innocent Mburanumwe, warden for Virunga's southern sector, he told the BBC lava was flowing towards an area to the south of the volcano, where "many people" live.
"I was woken at 0345 [0145 GMT] by a loud bang, which I first thought was the sound of war," he added.
"I thought there was fighting again near our park station," he said, referring to the conflicts which have wracked eastern DR Congo.
"Then I saw the mountain was on fire with sparks flying."
Mount Nyamulagira has registered more than 35 eruptions since 1882, with the most recent before this weekend being in 2006.
Virunga was made a UNESCO World Heritage Site in 1979 and is also home to 200 of the world's 720 remaining mountain gorillas. The mountain gorillas reside further east from Nyamulagira, on the flanks of the Mikeno volcano.