A rescue mission has been launched to keep a determined elephant seal off a Northern California highway that it has repeatedly tried to cross, causing traffic chaos.

California Highway Patrol spokesman Officer Andrew Barclay said callers first reported the 35-and-a-half stone mammal was trying to climb the central reservation wall of Highway 37 near Sears Point in Sonoma.

US Fish and Wildlife Service crews and police managed to usher the adult seal back into San Francisco Bay - but instead of swimming away, it got back on dry land at least twice.

"Every time we got her in the water, she circled back and tried to climb out again," Officer Barclay said.

Most of those trying to help the seal left the area after she got back in the water and the tide lowered, decreasing her chances of reaching land, but police were patrolling the area overnight in case the mammal made another crossing attempt.

The Sausalito-based Marine Mammal Centre and San Pablo Bay National Marine Sanctuary were also helping with the rescue.

Mammal centre spokeswoman Laura Sherr said the seal, which did not seem to be hurt, probably became lost and confused after swimming up the wrong waterway.