A multi-agency manhunt has been launched to find a suspect who allegedly gunned down a policeman during a traffic stop in California.

The heavyset, chain-wearing suspect, who is believed to have killed 33-year-old Ronil Singh, Stanislaus County sheriff Tom Letras said.

Authorities in California and other states are now on the lookout for the man ho was seen in surveillance photos at a convenience store before the shooting.

The heavy man with short, dark hair, a silver chain, jeans, dark T-shirt and a dark jacket with white Ecko brand patches on the shoulders is considered armed and dangerous, the California Highway Patrol warned.

Mr Singh was shot a few minutes after alerting colleagues that he was pulling over a grey pick-up truck with no licence plate in the small California town. He was pronounced dead at hospital.

A truck, which is believed to be the one stopped at the crime scene, was later found in a garage about 4 miles from the shooting and is being examined, police said.

Mr Singh joined the Newman police force in 2011. He was reported to be a native of Fiji and the father of an infant son.

“He was living the American dream,” said Stanislaus County Sheriff’s Department Deputy Royjinder Singh, who is not related to the killed officer but knew him. ”He loved camping, loved hunting, loved fishing, loved his family.

Mr Singh is survived by his wife, Anamika, and his five-month-old son, authorities said.

Outgoing California Governor Jerry Brown offered condolences to Mr Singh’s family and said flags at the state’s Capitol building would fly at half-staff in his honour.

“Our hearts are with the entire community of Newman and law enforcement officers across the state who risk their lives every day to protect and serve the people of California,” Mr Brown said.

In a photograph released by the Sheriff’s Department, Ronil Singh is shown smiling at a patrol car with a dog (AP)

The New York Police Department also paid its respects to the slain officer.

“We mourn the loss of this hero and send our condolences to his friends, family, and colleagues,” it tweeted.