PHILADELPHIA (Reuters) - Some 500 employees of a sprawling Boeing Co. facility in a southern suburb of Philadelphia were evacuated Tuesday morning after a threat was telephoned to the plant, authorities said.

Damien Mills, a company spokesman, declined to reveal the nature of the threat but said the company immediately notified local police and evacuated the workers - part of the 6,200 employees who make CH-47 Chinook helicopters and the V-22 Osprey aircraft at the facility.

"Regular operations continue at most of the plant," he told Reuters. Police were investigating the threat.

He said the evacuated employees are standing in a safe zone in the area of the plant, which is located in Ridley Township, south of Philadelphia in Delaware County.

The evacuation comes amid tensions surrounding the anniversary of the September 11, 2001, attacks, when nearly 3,000 people were killed by airliners hijacked by Islamist militants.

It also coincides with heightened concerns about workplace violence, including an episode in New York last month when a man shot and killed a former co-worker near the Empire State Building and an incident in suburban New Jersey when an employee shot two co-workers before taking his own life.

(Reporting by Dave Warner; Editing by Eric Beech)