Multiple airlines with flights to multiple US airports were at risk, according to a western counterterrorism official. Another official refused to identify the airlines because they were still being notified of the threat but referred to them as the "usual suspects." In the past, US cities with terrorism threats or plots have included Washington, New York, Boston and Los Angeles. Airlines whose planes were hijacked on Sept. 11, 2001, were United Airlines and American Airlines. British Airways has also dealt with numerous threats in recent years.

"These measures will continue to assure that our aviation system remains safe and secure," Chertoff added. "Travelers should go about their plans confidently, while maintaining vigilance in their surroundings and exercising patience with screening and security officials."

At US Northern Command, the military headquarters established in response to the Sept. 11, 2001, attacks to improve coordination of the defense of U.S. territory, spokesman Sean Kelly declined to comment on any precautionary steps taken in response to the heightened threat levels.

"It is inappropriate to speculate or comment on any current operational activities or discuss future force protection measures," he said.

Prime Minister Tony Blair's office said in London that the prime minister, vacationing in the Caribbean, had briefed President Bush on the situation overnight.

There was no immediate public reaction from the White House. Bush is spending a few days at his ranch near Crawford, Texas.

The Homeland Security Department devised the alert system after the Sept. 11 attacks. The last time the US government raised the terrorist risk here to orange, or high, was in July 2005 after the subway bombings in London. It was lowered to yellow a month later, the elevated risk status that has been the norm since the system was created.

US authorities, including the Transportation Security Administration, planned a news briefing early Thursday.

In London, Britain's Home Secretary John Reid said the alleged plot was "significant" and that terrorists aimed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."

Police arrested a number of people overnight in London after a major covert counterterrorism operation that had lasted several months, but did not immediately say how many. Heathrow airport in London was closed for most European flights.

The national threat level in Britain was raised to critical - a warning level that indicates the likelihood of an imminent terrorist attack. The threat rating was posted on the Web site of Britain's MI5 - the British domestic spy agency.