Images showed a massive grey cloud billowing forth from the volcano on the island of Sicily, beginning about 11:24 am local time (0924 GMT), according to the National Institute of Geophysics and Volcanology (INGV).

Surveillance cameras showed "a pyroclastic flow probably produced by a collapse of material from the northern flank of the Southeast Crater", the agency said.

A pyroclastic flow occurs when volcanic rock, ash and hot gasses surge from volcanos. They are extremely dangerous.

The explosive activity "had transitioned to a lava fountain", INGV said, with the plume of ash expected to dissipate towards the southwest.

The president of the region of Sicily, Renato Schifani, said experts had assured him there was "no danger for the population", with the flow not having passed the Valley of the Lions, an area frequented by tourists.

Videos posted on social media appeared to show tourists hurrying down the side of the volcano, some taking photos, but AFP could not determine their authenticity.

"The partial collapse of the Southeast Crater, which generated an impressive eruptive cloud several kilometers high and a pyroclastic flow, is a phenomenon that we follow with extreme caution," Schifani said.

The head of the regional civil protection unit, Salvo Cocina, recommended that tourists avoid the area "in consideration of the potential evolution of the phenomenon".

A red alert issued for aviation authorities said the height of the volcanic cloud was estimated at 6.5 kilometres (more than four miles).

The nearby Catania airport was still in operation.