Extra security checks again delayed a British Airways (search) flight to Washington Dulles International Airport on Sunday as the United States entered a third consecutive week on a high state of alert for terrorists.

As the number of cancellations and delays mounted, the chairman and ranking Democrat on a key House committee endorsed a rapid assessment to plug gaps in U.S. defenses against terror attacks.

"The ports of L.A. and Long Beach are where 43 percent of the container traffic for America enters and exits, and yet we still don't have a good system of inspecting containers," Rep. Jane Harman of California said in a televised interview.

Appearing on the same program, the Homeland Security Committee (search) chairman, Rep. Christopher Cox, R-Calif., said the government is working on an assessment of vulnerabilities. "We're doing everything we can to speed that along, to hasten it, but we've got to prioritize," Cox said.

Thirteen flights on British Airways, AeroMexico and Air France have been canceled or delayed since New Year's Eve because of security fears. Air France canceled six flights between Paris and Los Angeles on Christmas Eve and Christmas Day.

The U.S. government raised the national threat level on Dec. 21 to orange, indicating a high risk of terrorist attack, and said threat indicators are perhaps greater now than at any point since Sept. 11, 2001 (search). Dulles International, point of origin for the Sept. 11 flight that terrorist hijackers crashed into the Pentagon, was the destination or takeoff point for eight of the canceled or delayed flights since Wednesday.

On Sunday, a British Airways flight from London to Dulles, canceled twice because of security fears, sat on the ground for more than three hours past its regular departure time to conduct security checks requested by the United States. In recent days, the flight was twice delayed, including on New Year's Eve, when 247 passengers were held inside the plane for more than three hours after it landed at Dulles.

U.S. officials "have requested to be supplied with extra information about the flight for security purposes before takeoff to Washington, the same as happened yesterday," said British Airways spokesman Paul Parry.

But for the second day, flight 223 made the eight-hour trip without further interruption landed at Dulles without incident.

British Transport Secretary Alistair Darling (search) said the decision to cancel the flights was justified on the basis of intelligence warnings. Darling advised travelers to expect an increased number of security alerts in coming years.

"I fear that for many years to come we are going to be living in an age where there is going to be a heightened state of alert. Sometimes it will be quite severe, at other times perhaps less so," Darling told the British Broadcasting Corp. program "Breakfast with Frost."

"We are going to have to get used to increased security at airports. From time to time that will be noticeable, and at other times maybe things will be going on behind the scenes," Darling said.

British Airways has not yet decided whether Monday's scheduled flight from London to Riyadh, Saudi Arabia, will fly. The airline canceled a Saturday flight to Riyadh and a Sunday flight in the other direction.