A US Airways flight from Shannon Airport has been given the all-clear after making an emergency landing at Philadelphia International Airport due to an unspecified threat.

"There was some type of threat made, but nothing specified," a spokeswoman for the Philadelphia Police Department said.

Rather than following the usual landing procedure, the passengers were isolated and screened, and the luggage was screened, police said.

Flight 777 landed safely and without incident at its intended destination, airline spokesman Andrew Christie said.

"We were aware of a possible security issue with the flight and out of an abundance of caution taxied the aircraft to a remote location, where it was met by law enforcement and emergency personnel," Christie said.

All 171 passengers and eight crew members exited the plane by the stairs and were taken to the terminal by bus, Mr Christie said.

It is reported the all-clear was given after the passengers were screened and the plane searched.