More than two million people living on the banks of Lake Kivu in central Africa are at risk of being asphyxiated by gases building up beneath its surface, scientists have warned.

It is estimated that the lake, which straddles the borders of the Democratic Republic of Congo and Rwanda, now contains 300 cubic kilometres of carbon dioxide and 60 cubic kilometres of methane that have bubbled into the Kivu from volcanic vents. The gases are trapped in layers 80 metres below the lake's surface by the intense water pressures there. However, researchers have warned that geological or volcanic events could disturb these waters and release the gases.

The impact would be devastating, as was demonstrated on 21 August 1986 at Lake Nyos in Cameroon, in West Africa. Its waters were saturated with carbon dioxide and a major disturbance - most probably a landslide - caused a huge cloud of carbon dioxide to bubble up from its depths and to pour down the valleys that lead from the crater lake.

Carbon dioxide is denser than air, so that the 50mph cloud hugged the ground and smothered everything in its path. Some 1,700 people were suffocated.

"The lake was essentially like a bottle of beer that had been shaken up," said Professor George Kling, of the department of ecology and evolutionary biology at Michigan University. "When you opened it, carbon dioxide bubbled up, and the beer frothed over. A glassful is OK. A lakeful is deadly."

Kling has since turned his attention to Lake Kivu, which is more than 3,000 times the size of Nyos and contains more than 350 times as much gas. More worrying is the fact that the shores of Kivu are much more heavily populated. About two million people live there, including the 250,000 citizens of the city of Goma.

Mount Nyiragongo, near Goma, erupted in 2002 and lava streamed from it into Lake Kivu for several days. On this occasion there was no disturbance of the lake's deep layers of gas and no deadly outpouring of carbon dioxide or methane. However, Kling has warned - in the journal Nature this month - that in the event of another eruption the region may not be so lucky again.

Indeed, the impact would dwarf the disaster that struck Nyos. "Kivu is basically the nasty big brother of Nyos," Kling told Nature.

The source of Kivu's problems stems from carbon dioxide that has bubbled up through the lake bed from molten rocks below. The region - in Africa's Great Rift Valley - is a centre of volcanic activity. In addition, some of this carbon dioxide has been converted by bacteria in the lake into methane. Hence the accumulation of both gases.

According to studies by researchers at the Swiss Federal Institute of Aquatic Science and Technology, there was a 10% rise in carbon dioxide concentration, and a 15-20% increase in methane concentration in Kivu between 1974 and 2004. At the same time, plankton fossils on the lake's bed have revealed several massive bouts of biological extinctions in Kivu over thousands of years. However, it is impossible to say if a new one is imminent, researchers told Nature.

At the same time, engineers are trying to tap Kivu's rich supplies of methane - by lowering pipes from floating platforms down to its holding layers and siphoning off the gas. This could then be burnt and used as a source of industrial and domestic energy.

Several projects have been established, though only one is currently generating electricity - albeit sporadically - for the Rwandan grid. Another platform sank last year shortly before it was scheduled to begin production.

Tapping Kivu's methane could, theoretically, reduce the risk of a deadly eruption, say engineers. However, scientists have also warned that tampering with the lake's gases also carries a risk of triggering a disaster.