WASHINGTON Air industry officials said delays were easing but could continue for days, following news that U.K. authorities had arrested 21 people linked to a plot to set off bombs aboard flights bound for the U.S.

Flight delays continued to hamper air travel for passenger airlines, and thereby either delay or shift some of their freight shipments to freighter aircraft or other transport modes where possible. But freight industry officials said the overall impact was mild so far.

James May, president and chief executive of the Air Transport Association, told reporters on Thursday that the plot and subsequent security crackdown at numerous airports here and abroad was "having a dramatic impact on the airlines."

But he said flight schedules improved from hours of delays earlier in the day to "much closer to normal" as the day continued. Still, he cautioned that delays could develop anew as extra security measures spread to more airports in coming days.

U.K. authorities imposed a maximum security level on air operations, one reserved for risk of imminent attack. That included banning all carry-on luggage on flights leaving U.K. airports, shifting more into airplane bellies and crowding space for air freight. The sudden rule change backed up passengers at security checkpoints and triggered widespread delays.

British Airways, the largest airline using the U.K. airports and one of the world's top cargo carriers, for much of the day canceled all its inbound and outbound short-haul flights that last under three hours. It continued long-haul service with delays out of London's main Heathrow airport, and halted new cargo bookings before Aug. 14.

In the U.S., the Transportation Security Administration applied its highest risk rating to U.K. flights, and raised the security level throughout the country to a slightly lower level for other air travel.

U.S. Homeland Security Secretary Michael Chertoff said those planning the attacks were "getting close to the execution phase" before the arrests. CNN reported a government official said the terrorists wanted to hit flights to major destinations of New York, Washington, D.C., and California.

The heightened alert level for aviation has not spilled over to other modes, according to spokesmen at the Coast Guard and Customs and Border Protection. Customs issued a press release stating that officers will perform intensive passenger screening for flights originating in the U.K. and bound for the U.S. Inbound passengers also will be subject to more intensive screening upon arrival.

Customs will work to complement measures being taken by the Transportation Security Administration for domestic aviation. Homeland Security Secretary Michael Chertoff said early Thursday that TSA "is coordinating with federal partners, airport authorities and commercial airlines on expanding the intensity of existing security requirements."

In a statement issued Thursday morning, the Coast Guard said, "There is no information at this time suggesting the current terrorist threat extends into transportation sectors other than aviation. The Coast Guard has no plans to change the maritime security level at this time. We are continuing to monitor the maritime environment and work with our security partners to ensure appropriate measures are in place."

On the air side, U.S. authorities did not ban carry-ons, but did prohibit them from including any liquids and gels amid reports the plotters intended to use liquid explosives to blow up jetliners. And with passengers putting more of their bags under the cabins, May said the industry had to work out adjustments with the Federal Aviation Administration to the normal weight and balance formulas for cargo versus passenger loadings.

He also said the number of flight cancellations mainly out of the U.K. had been less than initially expected. He said 25 U.S. airports added extra gate checks, mainly for U.K.-bound flights, on top of the usual security checkpoints, and that others would do so in coming days along with more use of random security checks. The governors of Massachusetts and California called up National Guard forces to help with airport security, and some airfields used police forces to augment federal security screeners.

It was not clear how long the new rules would remain in effect. The ATA's May said he expected the measures to be temporary but had not received any such word from government officials. Along with flight delays from U.K. airfields the ripple effects spread to other airports than Heathrow - the events pushed some forwarders' freight shipments to air freighters that normally go into widebody passenger planes bellies.

Germany's Lufthansa airline said its cargo operations - also some of the largest in the world - would continue out of the U.K. "with the use of our freighter fleet via East Midlands, and enhanced truck operations from Heathrow."

Cargo that had been booked on delayed passenger flights "will be routed on to truck and freighter services where possible, and will be re-booked for new flights ex-Germany," Lufthansa said.

There were also reports of flights taking the air to meet schedule while many passengers were still stuck in backed-up security lines.