The Homeland Security Department said the directive, which is effective immediately, will further enhance security on commercial and cargo aircraft flying to, from and over the United States.

"We are asking international air carriers to take this protective action as part of our ongoing effort to make air travel safe for Americans and visitors alike," Homeland Security Secretary Tom Ridge said in a statement.

"I have said that we will take specific steps to increase security whenever necessary, and with this action we are doing just that," he added.

Homeland Security spokesman Dennis Murphy said the move will apply to specific flights "based on specific information" whenever it surfaces.

"We will then notify the carrier that based on information we received, we require a law enforcement officer to be on the plane," Murphy said. The directive contemplates that armed officers from the country of the airline’s ownership would be aboard.

A senior intelligence official said earlier this month that analysts were particularly concerned about the threat of Sept. 11-style attacks, in which terrorists would use hijacked airliners as weapons.

The directive comes in the form of three emergency amendments to air security regulations involving cargo planes, passenger planes and airliners passing over U.S. airspace.

There are thousands of international commercial and cargo flights daily involving U.S. airspace and hundreds of international carriers.

The Bush administration raised the terrorism alert level to orange, or high, on Dec. 21 and Air France canceled six flights between Paris and Los Angeles on Wednesday and Thursday, following security discussions between U.S. and French officials.

"What we are saying here is we expect this level of cooperation from all nations," Murphy told The Associated Press. "This step is in case we might not get that same level of cooperation that we’ve received thus far from our closest allies. We anticipate the same level of cooperation from all air carriers that fly to and out of the U.S."

In a news release, the Homeland Security Department said it will continue to conduct checks on passengers and crew of flights entering and leaving U.S. airspace, and will analyze threat information related to those flights.

When intelligence information warrants, it said, the government will direct additional security requirements for specific flights, including protection by law enforcement officers where warranted, it said.

Britain said yesterday it tightened security for trans-Atlantic flights and suggested it might put armed sky marshals on some planes. The United States already places armed security officers on certain flights.

"The last few days have seen the United States increase their general threat and security levels, and what we are proposing is a proportionate and appropriate level of response at a time when the threat to both our countries and around the world remains real and serious," Britain’s top law enforcement official, David Blunkett, said.

British airline pilots expressed concern over the measure. The British Airline Pilots Association said it would advise members who were not comfortable with the initiative not to fly their planes.

"We cannot agree with the government’s decision to put armed guards on aircraft as we believe this will do more harm than good." Balpa general-secretary Jim McAuslan said. "We do not want guns on planes."

Britain announced in December last year that it was training undercover armed police officers to act as sky marshals and would use them "where and when we believed it necessary."

Also in December last year, a government-commissioned report recommended that marshals be placed first on trans-Atlantic flights. If that proved successful, the program should be extended to most flights, said the report by former Conservative Party lawmaker Sir John Wheeler.

Transport Secretary Alistair Darling today said that the decision to deploy air marshals was justified, though he did not say if they would be armed.

"We are unfortunately faced with a world where there is a heightened alert and it’s necessary to deploy a whole range of measures," such as screening bags and passengers, Darling told the British Broadcasting Corp.