All Texas air travelers should get to their airports earlier than usual Thursday after security was tightened at Dallas-Fort Worth International and Houston Bush Intercontinental airports.

Fort Worth-based American Airlines, Houston-based Continental Airlines and suburban Chicago-based United Airlines were specific targets of the plot, two counterterrorism officials said Thursday, speaking on condition of anonymity because of the sensitivity of the situation.

American canceled six flights as of Thursday morning between London's Heathrow Airport and the U.S. cities of Boston, Chicago and New York.

From Dallas/Fort Worth, American serves London's smaller Gatwick Airport, where flights continued to operate, but with delays.

American told passengers that they can get refunds or travel vouchers in exchange for cancelled flights to or through the United Kingdom. The airline will also wave change fees for rescheduled travel, but any changes or refunds must be made by Sept. 1.

Dallas/Fort Worth International Airport officials said they were in contact with the TSA to see what changes in security would be required.

At D/FW, where American is based and hubbed, airport spokesman Ken Capps said extra police and K-9 units had been mobilized. Those units are patrolling terminals and parking garages.

"The Airport is bringing in extra officers, but no specific details will discussed on additional security precautions or enhancements until we have additional information about the developing impact of the UK situation on the United States," D/FW said in a statement.

"As always, the safety of our passengers is at the top of our list, and DFW remains at a high state of alert every day - just as it has done for several years," the statement said.

Officials encouraged passengers to continue their flight plans unless otherwise advised, and to be on the watch for "unusual behavior."

Passengers will be barred from carrying liquids and gels aboard flights, including beverages bought at airport concessions stands. U.S. security officials explained that only liquids emerged as a risk from the investigation in Britain.

The security was tightened before dawn Thursday after British officials broke up what they described as a terror plot.

British officials said the plan was to simultaneously blow up several jetliners en route to the U.S. using explosives smuggled on board in carry-on bags.

Similar measures have been taken at Bush Intercontinental in Houston, where Continental Airlines is based and hubbed. Airport spokeswoman Marlene McClinton urged domestic travelers to arrive two hours before departure, and international travelers to allow three hours.

U.S. officials have banned all carry-on liquids and gels, including beverages bought at airport concessions stands. Infant milk and formula are exempt from the ban if an infant is with an adult passenger. Also exempt are insulin and prescription drugs when the name on the label matches that on the boarding pass, Capps said.

The Dallas Morning News and the Associated Press contributed to this report.