Washington ? The United States will ensure that other governments enforce a new American requirement placing armed law enforcement officers on some flights to prevent hijackings, Homeland Security Secretary Tom Ridge said Monday as the nation headed into the New Year’s holiday with terror threats high.

Ridge also assured Americans who might be concerned about holiday air travel that aviation in the United States since the Sept. 11, 2001, terrorist attacks “has risen to new heights of security.” He encouraged Americans to continue with their holiday plans, even amid the orange-alert level, or high-alert status, put in place more than a week ago.

“The full force of Homeland Security all across this nation is at work to keep you safe,” he said.

The orange alert will stay in effect through the holidays and possibly beyond, Ridge said. Officials have not seen a reduction in air travel since the alert.

The new directive Ridge outlined Monday requires selected international flights that cross into U.S. airspace to carry an armed law enforcement officer aboard. The Homeland Security Department will require such officers on flights where intelligence information leads to a specific concern about that airplane, department spokesman Dennis Murphy said.

“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.

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

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

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

No longer voluntary

Ridge said the United States, like any nation, had the right to forbid foreign airlines from entering its air space unless they complied with the new requirements.

“Ultimately, a denial of access is the leverage that you have,” Ridge said.

Aviation security experts said the announcement marked 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 U.S. market to foreign carriers.

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

Some passenger lists are reviewed beforehand, he said.

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.

Nations willing to help

Some international airlines said Monday they would cooperate with the new U.S. requirement. Others, including airlines in Canada and Germany, said they already were using armed marshals on some flights.

Britain said Sunday it had tightened security for trans-Atlantic flights and suggested, as it has in the past, that it might put armed sky marshals on some planes.

A British Airline pilots association, however, expressed concern, saying it believed armed guards on aircraft would do more harm than good.

KLM Royal Dutch Airlines said Monday that introducing armed marshals on trans-Atlantic flights was among several new security measures it was discussing with the Dutch government.

In Russia, Aeroflot spokeswoman Irina Dananberg said the airline was ready to meet the U.S. request once it was made.