Olivia Newton-John makes heartfelt plea for missing beau

LOS ANGELES - A distraught Olivia Newton-John has issued a heartfelt plea for help to find her "treasured" boyfriend Patrick McDermott.

The Australian singing and acting star said she loved McDermott "very much".

Newton-John urged anyone with information about McDermott, who was last seen on a fishing boat off the coast of California on July 1, to contact US authorities.

"I'm hopeful that my treasured friend is safe and well, and I'm grateful to the officials who are working so hard to find Patrick, whom I love very much," she said in a statement.

"I ask anybody with information that could help to please come forward."

Newton-John has been in close contact with 48-year-old McDermott's family, friends and authorities in attempting to track down clues about his whereabouts.

LA-based Gavin de Becker, a friend of Newton-John's and one of America's best known security experts and private investigators, is lending a hand in the investigation.

Newton-John has used de Becker's expertise for her own personal needs in the past, including to protect her from infamous stalker Michael Perry in 1983 in Los Angeles.

The US coastguard and Los Angeles police department are baffled by McDermott's disappearance.

McDermott was last spotted on a fishing boat which left San Pedro Harbour, south of Los Angeles, on June 30.

The boat offered an overnight fishing expedition for anglers and McDermott was one of numerous fisherman who booked a place on the boat.

His car was found in the car park near the harbour almost two weeks after the boat returned, on July 11.

McDermott's belongings and fishing gear remained on the vessel.

US authorities are unsure if McDermott jumped off the boat out at sea or disappeared quietly once it docked. Authorities also have not ruled out foul play.

They have classed McDermott, an LA-based cameraman, as a missing person.

McDermott and Newton-John were in a relationship for nine years.

McDermott had been married previously and has a 15-year-old son from that relationship.

- AAP