The volcano of Ischia, a resort island famed for its thermal waters off the coast of Naples, could potentially erupt, Italian disaster experts said.

Guido Bertolaso, the head of Italy's civil protection agency, said that while Vesuvius was more commonly considered the nation's most worrisome volcano, Ischia, which last erupted 700 years ago, was experiencing a build-up of magma.

No eruption is imminent, but "if I had to say which is the volcano with the most loaded gun barrel, I'd say it's not Vesuvius but the island of Ischia", Mr Bertolaso said in Rome.

Ischia, a short hydrofoil trip from the port of Naples and the chic vacation island of Capri, is often overrun with Italian and foreign visitors seeking to ease their aches and pains in pools filled with thermal waters pumped in from the sea.

Ischia's "magma chamber is loading up", said Mr Bertolaso, whose agency's mandate includes both disaster relief and prevention. An eruption on Ischia "could be worse than a hypothetical Vesuvius eruption", he said.

While scientists detect no sign of an eruption, Ischia's volcanic potential is being monitored along with that of Vesuvius, the mountain that looms over Naples and destroyed Pompeii when it blew in 79 AD.

Vesuvius is a disaster planner's nightmare, since its slopes are officially inhabited by some 500,000 people, although perhaps as many as 150,000 more live in illegal, unregistered housing in the 18 hamlets clinging to its side, Mr Bertolaso said.

Considering that parts of crowded, chaotic Naples could be in the path of Vesuvius' fury, more than a million people might ultimately be ordered to evacuate in case of eruption, he estimated.

Seismologists have detected no signs of the quakes that are expected to rattle Vesuvius before any major eruption and could serve as a warning to evacuate, said Mr Bertolaso, whose agency includes vulcanologists as well as quake experts.

Italian volcano and earthquake experts are about to launch a several-year project to monitor and explore 13 largely unstudied undersea volcanoes in the Mediterranean, near Sicily and the southwest Italian coast, Mr Bertolaso announced.