<B>One week</B> after raising its terror alert level, the US government, on Monday, ordered foreign airlines to place armed marshals on selected flights to and from the United States to thwart attacks.

The Department of Homeland Security, which on Dec. 21 raised its terror alert to the second highest level, said it remained concerned about al Qaeda?s desire to conduct attacks against commercial airlines within the United States.

The department issued the aviation emergency orders to further boost security on aircraft flying to, from and over the United States.

When intelligence information shows signs of a potential threat on a flight, the department will require foreign carriers to place armed government law enforcement officers on particular passenger and cargo flights.

?These directives ... are part of our ongoing effort to make air travel safe for Americans and visitors alike,? Homeland Security Secretary Tom Ridge told a news conference.

The measures that go into effect immediately drew a mixed reaction, with some nations announcing their cooperation while some airline bodies criticized the order.

Last week, six Air France flights between Paris and Los Angeles were canceled on security concerns expressed by US intelligence. French authorities detained 13 people who had been booked on the flights but released them after finding no links to radical groups.

Some countries already place armed law enforcement officers on airplanes. The new measures give Homeland Security the authority to require armed security officers on international aircraft traveling in US airspace.

Armed air marshals disguised as passengers are already deployed on thousands of US airline flights each week in an effort to prevent another day like Sept. 11, 2001 when hijackers took control of four US passenger planes and flew them into the World Trade Center, the Pentagon and a Pennsylvania field.

Al Qaeda, led by Osama bin Laden, is blamed for the Sept. 11 attacks which killed about 3,000 people.

The United States has been seeking cooperation from other countries to help guard against a possible attack.

Britain appeared to anticipate the ruling, saying on Sunday armed sky marshals will be placed on some British passenger planes. But an international airline body, which argues security measures should be carried out on the ground before takeoff, was opposed to the decision, as were British airline pilots.

Mexico said it was prepared to collaborate by putting its police on identified flights, although it had not received any requests from its northern neighbor.

?If a specific situation arises, we will respond with Mexican personnel, specifically trained and in isolated situations,? Interior Minister Santiago Creel said. ?It isn?t something generalized for all flights.?

Ridge acknowledged that not all governments have programs to deploy marshals, and said the United States would help with training.

He said the new regulations served to warn governments they may be asked to provide marshals if necessary.

?It?s pretty clear that it is understood by our international aviation partners that the threat to passenger aircraft is an international challenge,? Ridge said.

?And all of us must work as closely together as possible to share information and act upon it to ensure the safety of our citizens, wherever that flight might originate or whatever its ultimate destination is.?

The United States would also continue to conduct checks on passengers and crew of flights entering and leaving US airspace, a Homeland Security statement said.

When it increased the terror alert level to ?orange? or ?high risk? of attack last week, the US government cited a sharp jump in intercepted communications about a possible attack during the Christmas and New Year?s holiday period.