A $1m reward has been offered in the US for information leading to the arrest of a fugitive ex-policeman suspected of three murders.

LA Police Chief Charlie Beck said the reward, raised from private donations and other sources, marks the largest sum ever offered in Southern California in a criminal investigation.

It was posted as law enforcement agencies across the region pressed on for a fourth day in their search for the suspect, ex-LAPD officer and US Navy reservist Christopher Dorner, 33.

Chief Beck described it as the most extensive manhunt ever mounted in the Los Angeles area.

He called the spate of revenge-driven violence Mr Dorner is accused of committing "an act of domestic terrorism".

"This is a man who has targeted those who we entrust to protect the public. His actions cannot go unanswered," the police chief said.

He said investigators were making progress but he declined to elaborate, saying they presumed that if Mr Dorner is still alive, he would be following media coverage of the manhunt closely.

Los Angeles Mayor Antonio Villaraigosa added, "Our dedication to catching this killer remains steadfast, our confidence in bringing him to justice remains unshaken."

The search for Mr Dorner has been focused in the snow-covered San Bernardino Mountains northeast of Los Angeles since a pickup truck belonging to him was found abandoned and burning near the popular ski resort community of Big Bear Lake on Thursday.

Police throughout the region have also chased down numerous unconfirmed sightings and dead-end leads.

One of the latest of those, prompted by calls from two individuals reporting they had seen someone resembling Mr Dorner, led police yesterday to a hardware store in LA's San Fernando Valley community of Northridge.

The store was evacuated and searched, but no evidence of  Mr Dorner's presence was uncovered, police said.

His last encounters with authorities came on Thursday in two Riverside County towns east of LA, police said.

He is accused of exchanging gunfire with a pair of police officers in Corona, injuring one, and later ambushing two policemen at a stoplight in Riverside. One of those officers was killed, the other wounded.

A rambling manifesto posted on Mr Dorner's Facebook page last week claimed he was wrongly terminated from the LAPD in September 2008 and vowed to seek revenge by unleashing "unconventional and asymmetrical warfare" on police officers and their families.

He was also named as a suspect in the killings of a campus security officer and his fiancée, the daughter of a retired Los Angeles police captain blamed in Mr Dorner's manifesto for his dismissal.

The couple, Keith Lawrence, 27, and Monica Quan, 28, were found shot dead last Sunday in their car on the top level of a parking structure in the city of Irvine, south of Los Angeles.

Mr Dorner had ended his military service two days earlier, but the Navy has not disclosed the circumstances of his discharge.