Four US police officers were shot dead by unknown gunmen near an air force base in Washington State in what police said was an ambush, US media reported.

The uniformed officers were sitting at a coffee shop in Tacoma, near the McChord Air Force Base, when two men stormed in and opened fire Sunday, broadcaster KING-TV reported.

Investigators believe the officers were targeted and that it was not a robbery. 'This was a targeted, selected ambush,' Pierce County police spokesman Ed Troyer told journalists.

Police have cordoned off streets around the scene and a manhunt was underway for the suspects, who fled the scene.

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