Security was sharply beefed up Thursday at airports in Los Angeles and other U.S. cities following the discovery of an alleged "mass murder" plot targeting airliners from Britain to the United States.

In response to the British announcement, the U.S. Department of Homeland Security raised the national threat level to its highest rung -- severe, or red -- for commercial flights from Britain, and to high, or orange, for all flights within the United States.

The British authorities said they foiled the plot and arrested 21 terrorist suspects.

There was no immediate announcement on which U.S.-bound flights had been targeted for in-flight destruction as part of the plot reported in Britain.

Five airlines operate 20 daily non-stop flights between London and Los Angeles -- Air New Zealand, American Airlines, British Airways, United Airlines and Virgin Atlantic Airlines.

Security measures at Los Angeles International Airport (LAX) are being beefed up even though officials have no information specifically naming the airport as a terror target, said a spokeswoman for Los Angeles World Airports, the agency that operates the LAX.

Passengers are advised to come to the LAX two or three hours earlier for domestic flights and three or four hours earlier for international flights.

"Due to the intensified security screening, security screening process is expected to take considerably longer than normal," said spokeswoman Nancy Castles.

Britain had raised its threat level to critical, the highest, to suggest a terror attack could be imminent. The alert remained in place even after the British authorities announced that 21 suspects had been detained.

As of 4 a.m. Pacific time (1200GMT), passengers in the United States are not allowed to carry any liquids, including drinks, hair gels and lotion onto airplanes, said a statement of the Department of Home Security.

The ban on liquids reflects the nature of the attack that had been planned in Britain, it said.

Also banned aboard airliners Thursday are electronic items, including key fobs of the type that can switch car alarms on and off.

"Travelers should also anticipate additional security measures within the airport and at screening checkpoints," said the statement, without indicating when the heightened measures would be relaxed.

Castles said the Los Angeles Airport Police Department was working with the federal Transportation Security Administration and other federal agencies Thursday morning "to implement a range of security measures appropriate to the heightened security".

Airport officials said travelers would see a beefed-up law enforcement presence at the LAX, including hundreds of uniformed officers and plainclothes, some with dogs.

In London, British Home Secretary John Reid has said the thwarted plot aimed "to bring down a number of aircraft through mid-flight explosions causing a considerable loss of life."

The British authorities said the terrorists planned to smuggle explosives onto some 10 airliners in carry-on luggage and to set them off in flight.

Reports said many international flights from and to Britain have been canceled.

(Xinhua News Agency August 11, 2006)