Forest fires swept through parts of the ritzy French Riviera for a second day Tuesday, devastating scenic woods and forcing thousands to be evacuated. At least four people have been killed.

Authorities speculated that the blazes -- some 30 broke out nearly simultaneously on Monday -- were caused by arson. Molotov cocktails were found in the region, local officials said.

The mayor of Roquebrune-Sur-Argens (search), Luc Jousse, called the fires "a new form of terrorism," although authorities were still investigating and had not definitely determined the cause.

Firefighters met with some success battling the fires near the western portion of the French Riviera, but at least one re-erupted Tuesday east of Draguignan, about 30 miles from the fashionable resort of Cannes, which was untouched.

Forest fires frequently devastate wide swaths of the Riviera backcountry during the summer. But these have been described as the worst in decades.

Clouds of yellow and gray smoke blew across the skyline halfway between Toulon and Nice (search). The region is a magnet for tourists and a favorite of painters.

About 60 homes were destroyed or damaged as flames burned across some 21,000 acres by Tuesday morning.

Police in Draguignan said they had detained a 30-year-old municipal employee from the village of Figanieres who is suspected of having started several fires.

Homes and campsites were abandoned as 20,000 people were evacuated, although many had returned by Tuesday afternoon.

Interior Minister Nicolas Sarkozy, visiting the ravaged area, said a Russian helicopter that can carry 12 tons of fire retardant was being called in. Italy also was sending firefighters and vehicles to help, according to the Civil Defense Department in Rome. The defense ministry also sent in 600 soldiers to help some 2,000 French firefighters.

President Jacques Chirac, in Papeete, Tahiti, promised that "the guilty will be sought out with extreme rigor" and "sanctions will be of an extraordinary severity."

Var (search) fire chief Col. Jacques Baudot said authorities had not definitely concluded that arson was to blame. But he noted that 28 fires were started Monday. "There is little probability that this is by chance," he said.

A fire in the same area just over a week ago burned some 24,710 acres of the Massif des Maures.