Four police officers have been shot dead in an ambush at a coffee shop in Washington state, officials say.

It happened on Sunday morning inside a Forza Coffee Company in Lakewood, about 60 kilometres south of Seattle.

The officers - three men and one woman - were sitting at their laptops when the shooter entered the café and opened fire. All were in uniform, wearing vests and had parked their marked cruisers outside.

There were two employees and a number of customers in the shop at the time, but no one else was injured.

Detectives believe the suspect deliberately targeted the officers and the attack was not part of a robbery.

The gunman is believed to have escaped on foot, and a $10,000 (U.S.) reward has been offered for information on his whereabouts.