OCEANSIDE, Calif. -- A worker suffered extensive burns Tuesday in a gas explosion at a North County construction site.

The 35-year-old man was trying to put gasoline into a chain saw at the work site in the 4300 block of Mesa Drive in Oceanside when fumes from the fuel exploded shortly before 4:30 p.m., according to police.

Bystanders sprayed water on the victim to put out the flames, which caused severe burns to about 50 percent of the lower half of his body, Sgt. Kelan Poorman said.

Medics airlifted him to UCSD Medical Center in San Diego.

The accident, which happened at the future site of a skateboard facility on the grounds of Martin Luther King Park, apparently occurred because the chain saw was hot from having been used a short time earlier, the sergeant said.