"Any sovereign government retains the right to revoke the privilege of flying to and from a country or even over their airspace," Homeland Security Secretary Tom Ridge said Monday. "So ultimately a denial of access is the leverage that you have."

Ridge also said the nation would remain at orange alert through the New Year's holiday and perhaps beyond. "We are as concerned today as we were yesterday," he said. "We'll be concerned as much this week as we were last week."

The new directive Ridge outlined Monday requires selected international flights that enter US airspace to carry an armed law enforcement officer aboard. The Homeland Security Department will require such officers on airplanes where intelligence information leads to a specific concern about that flight.

For months, US security officials have feared that al-Qaeda operatives would again hijack planes to use them as missiles. The most recent concerns centered not on domestic passenger flights, but on airliners or cargo planes that take off from overseas and cross over US airspace, either on their way to a US airport or to a foreign one.

American security officials will "be working with our aviation partners internationally to ensure that the kinds of protective measures we have requested actually" are implemented, Ridge said.

The Bush administration raised the terrorism alert level to orange, or high, on Dec. 21, citing nonspecific but credible threats of an imminent terrorist attack.

Air France canceled six flights between Paris and Los Angeles on Wednesday and Thursday, after security discussions between US and French officials.

French Transport Ministry spokesman Olivier Mousson said Monday that US security agents have inspected security at French airports since the United States raised its alert level.

Aviation security experts said the announcement marks a significant change in that, up until now, international security guidelines have been voluntary.

"In the past, no country has ever tried to impose on other countries any measures of aviation security," said Rafi Ron, president of New Age Security Solutions, a Washington-based consultancy, and the former security director for the Israeli Airport Authority.

Ron predicted that despite concerns about armed air marshals expressed by British pilots and others, the measure will be enforced without much resistance because of the huge importance of the US market to foreign carriers.

The next logical step will be for the international community to push for global aviation security standards, including mandated reinforced cockpit doors and better airport perimeter defenses.

Homeland Security officials said governments frequently set security and other standards for planes bound for their airspace.

Homeland Security reviews the passenger and crew manifests of all planes bound for US airspace, generally after the plane has taken off, because passenger lists are usually finalised only minutes before the plane taxis from the gate, department spokesman Dennis Murphy said.

Some passenger lists are reviewed beforehand, he said.