Mount Nyiragongo volcano erupted on Saturday, spewing red fumes into the night sky over the Democratic Republic of Congo's eastern city of Goma and sparking an exodus to neighbouring Rwanda.
The military governor of North Kivu province, of which Goma is capital, "confirmed the eruption of the Nyiragongo volcano ... at around 7:00 pm".
"Investigations are underway and people must follow the guidance of civil protection" units, General Constant Ndima said, appealing for calm.
The UN mission to DR Congo sent a helicopter "above the zone and confirmed the eruption", according to an internal note seen by AFP.
"However the lava is flowing towards Rwanda. The city of Goma and its environs are safe," said the mission, known by its French acronym MONUSCO, which has bases in Goma.
Power was cut in large parts of the city and hundreds of residents began leaving their homes.
Some headed out of the southern end of the city towards the nearby border post with Rwanda, while others headed west towards Sake, in the neighbouring Congolese region of Masisi.
"The sky has turned red," one resident, Carine Mbala, told AFP by telephone.
"There is a smell of sulphur. In the distance you can see giant flames coming out of the mountain.
"But there has not been any earthquake," she added.
"People are leaving or preparing to leave," another resident told AFP, as the streets began filling up, some carrying as many of their belongings as they could.
"I am taking the children and getting into the car. There is a risk that the lava will flow on Goma," another said.
For the moment, there is no sign of a flow of lava from the city, the correspondent said.
But the last time Nyiragongo erupted was January 17, 2002, killing more than a hundred people and covering almost all of the eastern part of Goma with lava, including half of the airport's landing strip.
Hundreds of thousands fled the city.
The deadliest eruption of the 3,000-metre high volcano was in 1977, when more than 600 died.
Goma sits on the mountain's southern flank and overlooks Lake Kivu.
The Goma region, which lies in North Kivu province, bordering Rwanda and Uganda, has six volcanoes, all higher than 3,000 metres.