SAN FRANCISCO -- For at least the 13th time since November, a portable toilet was set ablaze in San Francisco's Russian Hill neighborhood Tuesday morning, according to a fire dispatcher.

The fire was reported at about 3:20 a.m. near the intersection of Van Ness Avenue and Greenwich Street and was extinguished by arriving firefighters shortly afterward, the dispatcher said.

The police department was notified of the fire and an arson unit is investigating the incident, according to the dispatcher.

The fire is the latest in a series of alleged arson incidents involving portable toilets in the area.

A fire department spokeswoman said on Dec. 23 that there had been at least 12 portable toilets set ablaze since November, all in the vicinity of Russian Hill.

Fire Lt. Mindy Talmadge said in December that arson investigators were visiting the area and advising construction workers to keep their portable toilets away from homes and trees, and to lock them at night.