Police officer shot and killed, another injured, in Pennsylvania, US

A police officer has been shot and killed and another seriously injured when they were attacked during an early morning response to a domestic dispute in Pennsylvania.

A search is now underway for the gunman, who fled his home in Canonsburg, 40 km southwest of Pittsburgh, at around 3.30am (7.30pm AEDT) on Thursday.

It is not clear what triggered the shooting, which is understood to have broken out as soon as the police officers arrived at home, according to the New York Daily News.

Police told residents to stay inside their houses as they searched for the gunman throughout Canonsburg.

One of the officers died soon after the shooting and the condition of the other is not yet known.

Authorities said schools would open two hours later than usual as the investigation continued.