WASHINGTON — U.S. counterterror officials are warning of an increased risk of an attack this summer, given al Qaeda's apparent interest in summertime strikes and increased al Qaeda training in the Afghan-Pakistani border region.

On Tuesday, Homeland Security Secretary Michael Chertoff told the editorial board of The Chicago Tribune that he had a "gut feeling'' about a new period of increased risk.

He based his assessment on earlier patterns of terrorists in Europe and intelligence he would not disclose.

"Summertime seems to be appealing to them,'' Chertoff said in his discussion with the newspaper about terrorists. "We worry that they are rebuilding their activities.''

Other U.S. counterterrorism officials, who spoke on condition of anonymity, shared Chertoff's concern and said that al Qaeda and like-minded groups have been able to plot and train more freely in the tribal areas along the Afghan-Pakistani border in recent months. Osama bin Laden and his top deputy, Ayman al-Zawahri, are believed to be hiding in the rugged region.

"The threat coming out of there is very real, even if there aren't a lot of specifics attached to it,'' one of the officials said.

Chertoff's department has not made any move to increase the country's colour-coded terror alert system. Now, airlines are under orange -- or high -- alert, which is the second most serious level on a five-point scale. The rest of the country remains a step below at yellow, or elevated.

Chertoff said he is convinced that terrorists are regrouping. "Our edge is technology and the vigilance of the ordinary citizen,'' he said.

The secretary also urged Americans to be watchful for suspicious activities in the wake of recent terror incidents in England and Scotland. On June 29, two cars packed with gas cylinders and nails were discovered in London's entertainment district. The next day, two extremists smashed their flaming Jeep Cherokee into security barriers at Glasgow Airport's main terminal.

Al Qaeda and its sympathizers have shown an interest in summertime attacks. Some examples from recent years:

• In 2005, London faced two separate sets of transit attacks. The July 7 attacks on three trains and a bus killed 52. A second attack on July 21 was bungled when the detonators failed to light the explosives.
• Last summer, international counterterror authorities said they foiled a plot to use liquid explosives to take down roughly 10 U.S.-bound airliners leaving Britain.