Residents of Moroni, the capital of the Comoros islands, said on Sunday that they could see lava at the top of 2,361-metre Mount Karthala, one of the world's largest active volcanoes.
"When I saw Karthala glowing we came out to see what was going on but I really don't know, I'm worried," said 35-year-old Houmadu Saindou.
Karthala last erupted in April 2005, affecting as many as 40,000 residents and forcing thousands to flee in fear of poisonous gas and a possible magma flow.
It was the volcano's first eruption in more than decade on the archipelago nation, 300km off the coast of East Africa.
Volcano scientist Hamidou Nassor said Sunday's activity also amounted to an eruption but it was too soon to tell how dangerous this one would be.
"Effectively there is an eruption but we don't know yet if there will be any gas," he said.
"There is a risk of a lava flow but we don't know yet which direction. It is urgent that we get a helicopter there to assess what is going on."
African Union troops, who were in Comoros for the May 14 election, were expected to fly over the volcano on Monday morning to try to determine the direction of the lava flow in case evacuations are needed along its path.
Grande Comore, the main island
in the Comoros chain
Karthala's majestic crag and its fertile slopes covered in vanilla vine plantations form most of Grande Comore, the main island in the Comoros chain.
The islands have largely escaped major destruction from the volcano, which has erupted every 11 years on average over the last 200 years, but has had several close calls.
In November, Mount Karthala sent clouds of ash and sparks over the island, blanketing the capital Moroni and other villages in grey dust. Moroni is about 15km from Karthala's crater.