Homeland Security Secretary Michael Chertoff says the United States is stepping up its security measures, after authorities in Britain said they uncovered a major terror plot to blow up airplanes flying from Britain to the United States.

Homeland Security Secretary Chertoff said the United States has strong concerns about a plot that was "well on its way."

Homeland Security Secretary Michael Chertoff, left, and Attorney General Alberto R. Gonzales hold a press briefing, Thursday, August 10, 2006

"This was a very sophisticated plan and operation," said Mr. Chertoff.  "This is not a circumstance, where you had a handful of people sitting around you coming up with dreamy ideas about terrorist plots."

British authorities say a terrorist ring was plotting to blow up multiple commercial, U.S.-based aircraft, heading from Britain to the United States.

Chertoff said, in response, the United States has raised its threat level for flights coming from Britain to the highest level of alert, or red.  For all other in-bound international flights and domestic U.S. flights, he said, the threat level will be high, or orange.

He said there is currently no indication of any plotting within the United States, but the U.S. government wants to be prudently cautious.

"We believe that the arrests in Britain have significantly disrupted this major threat," he added.  "But, we cannot assume that the threat has been completely thwarted, or that we have fully identified and neutralized every member of this terrorist network."

He did not reveal details of what U.S. officials know about the plot in Britain or the investigation.  But another security measure he announced was a ban on air passengers carrying any liquid or gel substance with them on board the plane.

"One of the concerns we had was the possibility of bringing on board a number of components of a bomb, each one of which would be benign, but when mixed together, would create a bomb," he noted.

Chertoff declined to characterize the 21 people who have been arrested in Britain, saying that is a matter for British authorities.  But he indicated that U.S. authorities do suspect the al-Qaida terror network.

"This operation is in some respects suggestive of an al-Qaida plot, but because the investigation is still under way, we cannot yet form a definitive conclusion.  We're going to wait until all the facts are in," he said.

Al-Qaida masterminded the September 11, 2001 terror attacks on the United States.  Those attacks, nearly five years ago, relied on hijacked planes, which hit the World Trade Center, in New York, and the Pentagon, near Washington.  The fourth plane crashed in a field in Pennsylvania.