French and Italian scientists said on Wednesday they could not rule out another cataclysmic explosion by Vesuvius, the volcano that destroyed Pompeii in AD79.

Vesuvius has been capricious throughout history, blowing up in explosive clouds of dust and poisonous, superheated gas but at other times erupting more safely, rolling lava slowly down its conical flanks.

It has been dormant since 1944.

But in a paper published by the London science journal Nature, the Franco-Italian trio warn that this sleep will not be eternal.

They report that the volcano's magma chamber, deep beneath the surface, has been steadily rising over the past 20,000 years.

The shallowest reservoir now lies at a depth of eight to nine kilometres (five to 5.5 miles).

The chamber's depth determines the chemical composition of the magma and also the way in which its energy will be released, they explain.

Lead researcher Bruno Scaillet of the Institute of Earth Sciences in Orleans, France, said it was crucial to identify the composition of the magma lurking at the top of the chamber.

"If this magma is of a more acid composition, a type similar to the one which caused the Pompei eruption, you can can expect an extremely explosive, dangerous eruption," he told AFP.

In such an event, 700,000 people would be potentially at threat, he said.

"On the other hand, if the magma is of mainly basalt composition, as in the last eruption in 1944, it would be of a flow type, with streams of lava, and that would be far less destructive."

Geological signatures show that from about 20,000 years ago to the Pompeii eruption in AD79, Vesuvius had a treaclier magma associated with violent eruptions preceded by billowing clouds of suffocating dust and toxic gas, according to the paper.

But from 1631 to 1944, hundreds of safer "lava-flow" eruptions occurred, similar to what is happening at the moment at the Stromboli volcano on Sicily.

Scaillet warned: "Just because there has been this trend for 2,000 years doesn't mean we can rule out a return to more acid magma" capable of unleashing catastrophic eruptions.

"And because there hasn't been an eruption since 1944 doesn't mean we can say there won't be any more."

Vesuvius' comforting silence over the past 64 years could be attributable to a filling-up of the magma chamber, but equally it could mean that the chamber's ceiling has been sealed over, he said.

If so, the volcano could explode like a bolted-down lid on a pressure cooker.

"We cannot rule out this scenario," said Scaillet.