The United States remains on high alert over terror threats even after the busting of a plot targeting inbound flights from Britain, a senior U.S. official said on Sunday.

"We're concerned about other groups who may carry out attacks because they think we are distracted,"Secretary for Homeland Security Michael Chertoff said on ABC's "This Week."

"They also have to be concerned about copycats-other groups that may think they have the opportunity to carry out a plot," he said.

As a result, the administration will keep the nation on its highest threat alert for flights from Britain and on its second-highest alert for all other flights, he added.

Chertoff expressed confidence in the country's airport screening systems, saying that they are well-positioned to catch terrorists.

Apart from raising air travel alert levels, the Bush administration last week banned all liquids and gels on flights, including toothpaste, makeup, and suntan lotion, with baby formula and medication exempted.

Starting from last Friday, all U.S. air travelers are required to go through screening twice as the administration expanded security measures to all airports across the country.

It is not known how long the new airline security rules will remain in effect.