U.S. Homeland Security Secretary Michael Chertoff said on Wednesday that the foiled terror plot in London was apparently work of al Qaida.

"It was sophisticated, it had a lot of members and it was international in scope," Chertoff told a press conference.

"It was in some respects suggestive of an al-Qaida plot," he said, adding the final conclusion has yet been made.

Britain said 21 people had been arrested, including the alleged "main players," and the plot envisioned multiple explosions in multiple aircraft.

Chertoff said the plotters were in the final stages of planning before execution, though it was unclear whether the plot was linked to the upcoming fifth anniversary of the Sept. 11 terror attacks.

U.S. Attorney General Alberto Gonzales said the operation "could potentially kill hundreds."

FBI Director Robert Mueller also said the plot bears "the earmarks of an al Qaida plot."

Earlier in the day, the Bush administration raised the threat level for flights from Britain to "red," designating a severe risk of terrorist attacks.

All other flights, including all domestic flights in the United States, were put under an "orange," alert -- one step below the highest level.

The administration also banned all liquids and gels from flights, including toothpaste, makeup, suntan lotion, but baby formula and medicines were exempted.

Source: Xinhua