RWANDA'S interior minister on Friday warned residents of the northwestern town of Gisenyi to stay on alert for a possible evacuation order because a nearby volcano still posed a threat.

On Thursday, Mount Nyiragongo, across the border in the Democratic Republic of Congo, erupted, razing much of the town of Goma and prompting several hundred thousand people to flee to Gisenyi.

"The town of Gisenyi is potentially threatened by a lava flow from the eastern slope of the volcano," the minister, Jean de Dieu Ntihurungwa, told AFP.

Experts also warned of an explosive gas risk caused by the lava flowing into Lake Kivu, which straddles the border between the Democratic Republic of Congo and Rwanda.

They said molten rock, which erupted from Mt. Nyiragongo, is stirring up reserves of methane that lay desolved in the lake.

Reports say about 40 people may have died as a result of the eruption of Nyiragongo, which began spewing lava early Thursday.