Aug. 10 (Bloomberg) -- The U.S. raised its terror alert to the highest level for flights from the U.K. after the discovery of a ``well-advanced plan'' that suggested al-Qaeda was plotting to use liquid explosives and detonators disguised as electronic devices to blow up jetliners in midair, officials said.

The threat level was raised to red, or ``severe,'' for commercial flights to the U.S. from Britain and to orange, or ``high,'' for domestic routes, Homeland Security Secretary Michael Chertoff said at a news conference in Washington.

The U.K. raised its terror alert to ``critical,'' the highest category in a five-point scale, indicating an attack is expected ``imminently,'' according to the Home Office Web site.

Chertoff said U.S. airline security is being tightened ``to make sure there are no remaining threats.'' Carry-on luggage can't contain hair gels, lotions or any liquids, including beverages. U.S. marshals will be posted on flights from the U.K., and canine teams and explosion detection technology will be deployed at international arrival areas, he said.

The arrest in the U.K. of 24 terror suspects ``has significantly disrupted the threat,'' Chertoff said in a statement ahead of the briefing. ``But we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted.''

All those arrested were British citizens, Charlie Allen, head of the Homeland Security Department's intelligence operations, told reporters on a conference call this afternoon.

Attacks Imminent

U.K. authorities arrested the plotters because they learned the suspected terrorists might do a test run of their plan in the next couple days and carry out their plan soon thereafter, said a U.S. intelligence official who declined to be identified.

On the afternoon conference call, Homeland Security Deputy Secretary Michael Jackson said the plotters were ``close to the point of being able to execute.''

Information on the timing was based partially on intelligence gathered after an arrest made in Pakistan in the last 24 to 36 hours, the U.S. intelligence official said, declining to provide further details.

Chertoff said the U.S. officials raised the terror level because of the possibility the plotters might inspire ``copycats.'' Homeland Security spokesman Russ Knocke said ``at this time there is no indication of plotting in the U.S.''

Chertoff and other top U.S. security officials said the plot points to al-Qaeda.

`Earmarks of al-Qaeda'

``This had the earmarks of an al-Qaeda plot,'' FBI Director Robert Mueller said. Attorney General Alberto Gonzales said, ``A plot of this sophistication is suggestive of al-Qaeda tactics.''

Chertoff said the plot is ``reminiscent'' of a foiled al- Qaeda plan in 1995 to detonate bombs on 11 airliners flying to the U.S. over the Pacific.

The U.S. intelligence official who declined to be identified said those arrested were plotting to blow up as many as 10 flights in midair.

Chertoff said one concern of authorities was that the plotters would bring in their carry-on luggage ``benign'' components that could be assembled into a bomb.

Jackson said the plotters were figuring out which airlines to target when they were arrested.

United, American, Continental

``In recent days, the focus seemed to bear in on carriers that have direct, non-stop flights between the U.S. and the U.K.,'' he said. United Airlines, American Airlines and Continental have such flights, Jackson said.

While U.K. authorities tracked the plotters for some time and communicated with their U.S. counterparts, the U.S. investigation intensified at the end of July when it became evident of the planes' destination, the intelligence official said.

President George W. Bush today said the plot is a ``stark reminder'' that the country remains at war with terrorists. ``This country is safer than it was prior to 9-11,'' Bush said upon arrival at a factory in Green Bay, Wisconsin, where he was scheduled to give remarks on the economy. ``But obviously we're still not completely safe.''

California, Massachusetts and New York were among U.S. states that increased security at airports.

California Governor Arnold Schwarzenegger and Massachusetts Governor Mitt Romney called on the National Guard to help with screening at airports in their states, and New York Governor George Pataki said travelers at John F. Kennedy International and LaGuardia airports should expect delays and canceled flights due to increased security.

Subways, Trains, Buses

Public transit agencies added patrols on subways, trains and buses, with New Jersey Transit adding police patrols at stations and on trains and buses throughout the state. The Washington Metropolitan Transit Authority that runs the Metro subways closed bathrooms and increased sweeps in select stations.

The color-coded U.S. Threat Advisory system was instituted after the Sept. 11 attacks. It ranges from green, or ``low;'' to blue, or ``guarded;'' to yellow, or ``elevated;'' and then to orange and red.

Bush approved raising the threat level yesterday upon the recommendation of Chertoff and other security officials, White House Press Secretary Tony Snow told reporters traveling with Bush on Air Force One.

The president was kept apprised of the investigation, speaking with U.K. Prime Minister Tony Blair in a 47-minute teleconference on Aug. 6 and again by telephone yesterday, Snow said.

Snow said Americans shouldn't be worried about taking flights. ``It is safe to travel,'' he said. ``There are going to be some inconveniences.''

Snow said he didn't know how long the heightened alert level would last.

To contact the reporters on this story: Nadine Elsibai in Washington at nelsibai@bloomberg.net. Jeff Bliss in Washington jbliss@bloomberg.net.

Last Updated: August 10, 2006 15:51 EDT