The U.S. government raised its threat warning to the highest level for commercial flights from Britain to the United States early Thursday in response to a terror plot disrupted in London. Terrorists had targeted United, American and Continental airlines, two U.S. counterterrorism officials said.

In addition to the highest alert for flights from Britain, the alert for all flights coming or going from the United States was also raised slightly. The government banned beverages, hair gels and lotions from flights, explaining only that liquids emerged as a risk from the investigation in Britain.

Multiple flights to several U.S. cities also were put on alert, U.S. Homeland Security Secretary Michael Chertoff said.

"We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted," Chertoff said.

It is the first time the red alert level, defined as designating a "severe risk of terrorist attacks," has been invoked in the U.S. Homeland Security warning system.

In addition to the red alert for flights from Britain, the alert for all flights coming or going from the U.S. was raised to the high, or orange, threat level "to defend further against any remaining threat from this plot," Chertoff said.

The government said it was banning beverages, hair gels and lotions from flights, explaining that liquids emerged as a risk from the investigation in Britain.

Chertoff said in a statement that "currently, there is no indication ... of plotting within the United States."

A U.S. law enforcement official said there had been no arrests in the U.S. connected to the overseas plot unraveled Wednesday evening.

Authorities believe dozens of people, possibly as many as 50, were involved or connected to the plot, a senior U.S. counterterrorism official said. The plan "had a footprint to al-Qaida back to it," the official said, speaking on condition of anonymity because of the sensitivity of the situation.

The plan involved airline passengers hiding masked explosives in carryon luggage. They were not yet sitting on a plane, but were close to traveling, the official said.

Another counterterrorism official said the investigation, which has been ongoing for months, was "very serious," and that U.S. intelligence had been working closely with the British.

It was not believed to be connected, however, to the 11 Egyptian students who disappeared in the U.S. more than a week ago before reaching a Montana college they were supposed to attend. Three have since been found, and the FBI said neither they nor the eight still missing were believed to be a threat.

Authorities have not yet arrested or detained all suspects believed to be involved in the plot, the second official said, also on condition of anonymity.

Chertoff said that, given the higher threat levels, U.S. transport authorities were coordinating with federal partners, airport authorities and commercial airlines to intensify existing security requirements.

"Due to the nature of the threat revealed by this investigation, we are prohibiting any liquids, including beverages, hair gels and lotions, from being carried on the airplane," he said.

Because metal detectors and X-ray machines cannot detect explosives, transport authorities deployed "sniffer" or "puffer" machines, which can detect explosives residue, at many but not all airport checkpoints.

Travelers should expect additional security measures at U.S. airports, but "should go about their plans confidently, while maintaining vigilance in their surroundings and exercising patience with screening and security officials," Chertoff said.

Multiple airlines with flights to multiple U.S. airports were at risk, a counterterrorism official said. 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, U.S. 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 also has dealt with numerous threats in recent years.

British Prime Minister Tony Blair, on holiday in the Caribbean, briefed U.S. President George W. Bush overnight on the situation, according to Blair's office in London.

There was no immediate comment from the White House or Bush, who was vacationing for a few days at his ranch near Crawford, Texas.

The U.S. Homeland Security Department devised the alert system after the Sept. 11 attacks. The last time the U.S. government raised the terrorist risk 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.

U.S. authorities 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.