US investigators are using intelligence from the UK to find out if people in the US had links to a suspected plot to blow up transatlantic planes.
Flights from the US to and from the UK are rated high risk
Homeland security officials voiced concerns that people with knowledge of any bomb plot might still be at large.
Police and security services in the UK arrested 24 people on Thursday, while US and UK air security was stepped up.
President Bush's homeland security adviser stressed the need to be sure of the extent of any threat within the US.
"There is no indication of plotting, but of course we'd be remiss if we weren't looking for connections between individuals here in the US and the plotters in the UK," Frances Townsend said.
"There are leads that the FBI is running, but there is no indication of operatives planning something big here in the United States."
Plan B fears
Intelligence services in the US and UK have long-established agreements to share information during security alerts.
According to an FBI spokesman, the US domestic intelligence agency is currently acting on a "new wave" of material provided by British police and security services.
The UK had been superb at sharing information about the associates, contacts and movements of those arrested, the spokesman said.
That information will be used to run checks on US-based suspects and to flesh out whether anyone under surveillance in the US had links to the UK plot.
Chief among US concerns is the prospect of a "Plan B" attack by US-based or other foreign operatives moved to action by the break-up of the UK cell, officials said.
"These are desperate, vicious people who have a good degree of sophistication, and they're out there, so if nothing else, the danger is that they're available for future operations," said Peter King, chairman of a congressional committee on homeland security.
"The more deadly threat is that there is a Plan B that they would be able to implement."
Airports on alert
One of the 24 suspects arrested in the UK was released without charge on Friday, but the rest remained in police custody.
The assets of 19 of those suspects have been frozen, and the Bank of England has published their names.
Passengers and baggage in the US are facing stiff checks
Those held are suspected of involvement in a plot to blow up transatlantic airliners, possibly using liquid explosives hidden in hand luggage.
Airport security in the US has been stepped up in response to the threat, with stringent luggage searches and restrictions of carrying liquids on board.
Flights heading to and from the UK remained classified as the highest level of risk.
Homeland Security Secretary Michael Chertoff said some of the more restrictive measures in place at US airports could be relaxed at some point soon.
"We're going to try to make this as simple and as easy as possible as quickly as possible," he said, adding that security officials needed to ensure adequate procedures were in place for screening liquids for explosive materials.