Airports in the United States are on alert for Pakistanis with "rope burns, unusual bruises or scars" that could have been received at terrorist camps run by Islamic extremists.

Inspectors at six of the nation's busiest airports, including Washington Dulles International, are on alert for travellers of Pakistani descent, including US citizens of Pakistani origin, as officials fear that potential terrorists are trying to sneak into the US, The Washington Times reported.

An internal bulletin of the Department of Homeland Security has warned the US Bureau of Customs and Border Protection (CBP) inspectors that potential terrorists who trained at camps in Pakistan might seek to return to the US between now and the November Presidential elections to carry out new attacks.

Law enforcement authorities confirmed yesterday that the bulletin directs "increased scrutiny" of passengers at Dulles, John F Kennedy International Airport in New York, Newark's Liberty Inernational Airport, Detroit Metropolitan Wayne County International Airport, O'Hare International Airport in Chicago and Los Angeles International Airport.

A Homeland Security official played down the alert, saying it was a "regular" memo to inspectors at six airports and the 22nd such memo this year.

But he conceded the Pakistani link, saying: "It basically told them to be on the lookout for passengers that may have had certain activities or lack thereof when travelling to and from Pakistan."

This, he said, is a regular occurrence, and there is no specific threat associated with it.

"At the same time," he said, "we asked inspectors to look at certain activities of travellers coming into the US."

The June 17 bulletin directs Federal inspectors to focus on foreign travellers, naturalised U.S. citizens or legal Permanent Residents (Green Card-holders) of Pakistani descent "who exhibit evidence of suspicious travel, including short trips to Pakistan not related to family or business."

The suspicious travellers are to be directed by primary inspectors at the six airports to secondary inspections, where  more detailed checks are made.

The bulletin noted that persons who trained at terrorist camps may exhibit certain features the inspectors might be able to readily identify, including rope burns on the arms and legs from rappelling training (moving down steep inclines as in mountaineering with double ropes), unusual bruises from obstacle courses, and wounds or scars that might have occurred during firearm or explosives training.

"US intelligence officials are concerned," according to the bulletin, "that persons trained in Pakistani camps could be intent on "committing illegal activities in the US."

Additionally, the Department of Homeland Security announced this week a dozen commercial airline pilots and crew members from foreign countries were banned from flying into the US after background checks showed suspected ties to terrorists or other criminal activity.

Homeland Security officials scrutinized 450,000 crew members who have flown into the US since March and found nine pilots with suspected terrorist ties, two crew members carrying fake passports and a third with an arrest record for assaulting a police officer.

The officials said the countries of origin of the flight crew members could not be released because of legal concerns.

Government officials also inspected 2.7 million truckers licensed to carry hazardous material and flagged 29 drivers with suspected links to terrorist operations.