A Southwest Airlines flight bound for Texas from Los Angeles with 143 passengers aboard was forced to land in Phoenix on Monday after a telephoned bomb threat and US fighter jets were diverted to monitor the situation, authorities said.

Southwest Airlines flight 2675 landed safely in Phoenix, the state capital of Arizona, at about 3:30 pm local time (2230 GMT), and all passengers on board were taken off without incident, the Phoenix Sky Harbor International Airport and Southwest said.

Phoenix police interviewed the passengers and bomb squad officers and dogs swept the aircraft. Police spokesman Sergeant Steve Martos later Tweeted that the search found "nothing of concern."

The Austin-bound flight was diverted at the request of the Los Angeles Police Department after an unidentified caller made a bomb threat against a commercial flight from Los Angeles to Texas, according to Southwest and the FBI.

Laura Eimiller, the FBI's spokeswoman in Los Angeles, said an investigation was under way to determine who was responsible for the threat.

While the plane was in the air, F-16 fighter jets were diverted from Davis-Monthan Air Force Base in Tucson, Arizona to "monitor the situation from the air," said Michael Kucharek, a spokesman for the North American Aerospace Defense Command (Norad), a joint US-Canadian operation.

"It was serious enough that we diverted aircraft from their original flight plan," Kucharek told Reuters.

The Southwest Airlines aircraft, a Boeing 737-700, was isolated at the airport away from the terminal after landing.

All the passengers were evacuated using airline steps and would be taken on to Austin "as soon as possible," Southwest Airlines spokeswoman Katie McDonald said.

Flights at Sky Harbor were arriving and departing as scheduled. - Reuters