With the terror alert level still at high, U.S. government officials announced Monday they are requiring armed, trained air marshals on some international flights that land in, depart from or fly over the United States.

The mandate is effective immediately, according to the Homeland Security Department (search).

"This is another in a long list of measures that we have taken in the last two and a half years to increase aircraft security," Homeland Security Secretary Tom Ridge (search) said at a news conference Monday. "This is an international challenge that we all have."

The directive's intent is to shore up security for commercial and cargo planes flying to, from and over the United States.

It suggests that armed law enforcement officers from the ownership country of a relevant airline be aboard and includes three emergency amendments to air security regulations involving cargo planes, passenger jets and airliners flying over U.S. airspace.

Ridge also assured Americans who may 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.

But Nevada is taking the additional step of closing airspace over the Las Vegas Strip (search) on New Year’s Eve, a homeland security adviser to Gov. Kenny Guinn told The Associated Press. A spokeswoman with McCarran International Airport said the ban — known as a temporary flight restriction or TFR — will be implemented from 9 p.m. to 3 a.m. on Jan. 1. About 300,000 people are expected to flock to the busy Strip this New Year’s Eve.

Special Agent Todd Palmer, an FBI spokesman in Las Vegas, said Monday that there were no specific threats against Las Vegas or Nevada. In addition, Homeland Security officials said it is not unusual for major cities to request a TFR during a heightened terror alert.

Chicago has had a TFR in place since the terror threat level was raised to orange, officials said, and New York City, Disney World and the sites of Bowl Championship Series games are expected to request their own.

Homeland Security spokesman Dennis Murphy said the move to require sky marshals 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.

An American 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 announcement comes a day after Britain said it might put armed sky marshals on some planes in an overall tightening of security for trans-Atlantic flights — some of which would be traveling to and from the United States. The United States already places armed guards on some of its 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," said Britain's Home Secretary David Blunkett (search), the Cabinet official responsible for law enforcement.

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 would continue to conduct checks on passengers and crews of flights entering and leaving U.S. airspace, and would analyze threat information related to those flights.

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

The Associated Press contributed to this report.