Ten days after the country went to orange or "high" terror alert, and a day after the U.S. began requiring certain U.S.-bound flights to carry armed security agents, Ridge told the CBS News Early Show that there is intelligence pointing to a possible threat to "major cities and large gatherings and critical infrastructure."
"It's a national alert and we're taking national precautions and using assets and people we have available at the federal level but working in coordination with the state and local officials," Ridge said. "I think the level of security this time around, within the United States, is absolutely unprecedented."
Security forces overseas were also on alert Tuesday. German police closed off streets around a military hospital in a Hamburg suburb following indications that Islamic extremists planned a car bomb attack.
CBS News Correspondent Randall Pinkston reports that in the United States, the extra security will mean metal detectors at New York City's Times Square celebration on New Year's Eve, as well as a no-fly zone over Las Vegas' famous strip.
Ridge said Monday the United States would remain at the second-highest alert level 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 Bush administration raised the terrorism alert level to orange, the second highest of five alert levels, on Dec. 21, citing nonspecific but credible threats of an imminent terrorist attack.
A new directive outlined by the Homeland Security Department on Monday requires selected international flights that enter U.S. 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. Homeland Security reviews the passenger and crew manifests of all planes bound for U.S. airspace.
The directive says that armed government officers from the country of the airline's ownership would be aboard, and they be equipped to prevent anyone from reaching the plane's cockpit and to communicate with the crew, Homeland Security spokesman Dennis Murphy said.
Most major world airlines, facing a ban from U.S. airspace if they do not place armed agents on flights highlighted by U.S. intelligence, said Tuesday they either were already complying or would do so if asked.
Britain declared the United Kingdom's willingness to deploy sky marshals, but stressed that "only the U.K. can authorize the placing of air marshals on U.K. carriers."
On Sunday, the British government said it had increased security measures for trans-Atlantic flights in response to the Code Orange alert in America and said sky marshals would be used "where appropriate."
U.S. officials say they have specific intelligence British-based airlines could be a target, and sky marshals have already been put aboard at least one Virgin Atlantic flight, reports Martin.
In Germany, Lufthansa spokesman Thomas Jachnow said the airline has been carrying sky marshals on some of its trans-Atlantic passenger flights since the Sept. 11 attacks by al Qaeda.
KLM Royal Dutch Airlines said introducing armed marshals on trans-Atlantic flights was among several new security measures it was discussing with the Dutch government.
Peter Coyles, spokesman for Transport Canada, said certain Canadian flights to the United States, including all to Ronald Reagan Washington National Airport, had carried armed law enforcement officers since shortly after Sept. 11.
In Mexico, Interior Secretary Santiago Creel said his nation will arrange for its own armed federal agents to travel on certain flights over the United States that represent a security risk.
In Russia, Aeroflot spokeswoman Irina Dananberg said she wasn't aware such a request had been made to the airline, but it was ready to cooperate.
French Transport Ministry spokesman Olivier Mousson did not say whether France would conform to the request, but said, "The French and the Americans cooperate totally in the struggle against terrorism. We work hand-in-hand."
Unarmed security agents had been aboard "Air France flights judged to be sensitive" since the attacks on New York and Washington in 2001, a spokeswoman for the French airline said.
The airline canceled six flights between Paris and Los Angeles on Wednesday and Thursday following security talks between U.S. and French officials.
Those cancelled flights led Homeland Security to move ahead with issuing the new rules requiring armed guards.
"With the recent threat reporting involving foreign flights, we had begun some private discussions with a couple of countries but we decided to take an organized effort and identify this need to all our international aviation partners," Ridge told the Early Show.
For months, U.S. 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 U.S. airspace, either on their way to a U.S. airport or to a foreign one.
Officials say it is possible al Qaeda is putting out false information about plans to hijack airliners in order to disrupt holiday travel, but no one is willing to take a chance the threats aren't real, reports CBS News Correspondent David Martin.