NEW YORK (AP) - U.S. air travelers poured out liquids, opened their bags for inspection and endured long waits Thursday as airports heightened security and some flights were canceled or delayed after the discovery of a terror plot aimed at airlines traveling from Britain to the United States. The airlines targeted included United Airlines, American Airlines and Continental Airlines Inc., two counterterrorism officials said Thursday, speaking on condition of anonymity because of the sensitivity of the situation. "This is very, very serious, this is the real deal," said Rep. Peter King, chairman of the House Homeland Security Committee. King said he was briefed on the case late Wednesday by Homeland Security Secretary Michael Chertoff, and "it was the most concerned I have ever heard him." American canceled three London-bound morning flights from Chicago, Boston and New York to accommodate delays at London's Heathrow airport, spokesman John Hotard said. To balance the cancellations, the airline also dropped three afternoon or evening flights from London to U.S. cities, Hotard said. The remaining 13 flights in each direction were expected to run from 1½ to 3½ hours late. The cancellations were due to scheduling delays and not because of possible threats to the flights, Hotard said. "We're going to be flying out of Heathrow today," he said. "It's just that because it's so congested right now." Most European carriers canceled flights to Heathrow because of the massive delays created by strict new regulations banning most hand baggage. The U.S. government raised its threat warning to the highest level for commercial flights from Britain to the United States and raised slightly the alert for all flights coming or going from the U.S. It is the first time the red alert level in the Homeland Security warning system has been invoked, although there have been brief periods in the past when the orange level was applied. Homeland Security defines the red alert as designating a "severe risk of terrorist attacks." The government said it was banning liquids including beverages, hair gels and lotions from flights, explaining only that liquids emerged as a risk from the investigation in Britain. Passengers complied at the Kennedy airport terminal housing British Airways, throwing those and other products into bins. At Dulles International Airport outside Washington, Homeland Security put up hastily printed signs warning passengers in all-red capital letters, "No liquid or gels permitted beyond security." The signs were taped up at ticket counters as well as the security checkpoints. Passengers dumped liquids into large trash cans that were being emptied every couple minutes. People threw away water bottles, juice boxes, makeup and even a bottle of tequila. At Dallas-Fort Worth International Airport, where American is based, extra police and dog units were sent out overnight to patrol terminals and parking garages, airport spokesman Ken Capps said. Passengers appeared to be moving smoothly through checkpoints, he said. At Chicago airports, the terror alert level was raised to orange, requiring all bags to be screened. Chicago aviation commissioner Nuria Fernandez said the only exceptions to the ban on liquids would be breast milk and fruit juices for small and nursing children. She urged passengers to arrive early for flights and to consider not bringing carryon luggage. Some passengers were surprised by the ban on flying with liquids at Hartsfield-Jackson Atlanta International Airport. Baby formula and medicine had to be presented for inspection. The terror plot and flight cancellations have "not yet" caused any disruption at Los Angeles International Airport, airport spokeswoman Nancy Castles said early Thursday. She declined to comment on whether airport officials have heightened security because of the raised threat alert, but said the airport typically followed any special directives from federal authorities. Delta Air Lines spokesman Anthony Black said Thursday morning that operations would continue normally and there would be no flight cancellations. But Delta was expecting delays on flights coming from the United Kingdom because of heightened security there, Black said.