NEW YORK - Flight delays piled up across the country Monday as thousands of air traffic controllers began taking unpaid days off because of federal budget cuts, providing the most visible impact yet of Congress and the White House's failure to agree on a long-term deficit-reduction plan.

The Federal Aviation Administration kept planes on the ground because there weren't enough controllers to monitor busy air corridors. Cascading delays held up flights at some of nation's busiest airports, including New York, Baltimore and Washington. Many operations were more than two hours behind schedule.

At one point, the delays were so bad that passengers on several Washington-New York shuttle flights could have reached their destination faster by taking the train.

Nearly a third of flights at New York's LaGuardia airport scheduled to take off before 3 p.m. were delayed 15 minutes or more, according to flight-tracking service FlightAware. Last Monday, just 6 percent of LaGuardia's flights were delayed.

The situation was similar at Washington's Reagan National Airport, in Newark, N.J., and in Philadelphia, with roughly 20 percent of flights delayed.

At airports, Monday is typically one of the busiest days, when many high-paying business travelers depart for a week on the road. The FAA's controller cuts - a 10 percent reduction of its staff - went into effect Sunday. The full force was not felt until Monday morning.

Travel writer Tim Leffel had just boarded a US Airways plane from Charlotte, N.C., to Tampa, when the flight crew had an announcement.

"They said: 'The weather's fine, but there aren't enough air traffic controllers,"' Leffel said. Passengers were asked to head back into the terminal. "People were just kind of rolling their eyes."

His flight landed one hour and 13 minutes late.

One thing working in fliers' favor Monday was relatively good weather at most major airports. A few wind gusts in New York, snow in Denver and thunderstorms in Miami added to some delays, but generally there were clear skies and no major storms.

However, the furloughs will continue for months, raising the risk of a turbulent summer travel season. And the lack of controllers could exacerbate weather problems, especially spring and summer thunderstorms.

There's no way for passengers to tell in advance which airport or flights will experience delays.

FAA officials have said they have no choice but to furlough all 47,000 agency employees - including nearly 15,000 controllers - because the agency's budget is dominated by salaries. Each employee will lose one day of work every other week. The FAA has said that planes will have to take off and land less frequently, so as not to overload the remaining controllers on duty.

Critics have said the FAA could reduce its budget in other spots that wouldn't delay travelers.

"There's a lot finger-pointing going on, but the simple truth is that it is Congress's job to fix this," said Rep. Rick Larsen, a Washington Democrat and member of the House aviation panel. "Flight delays are just the latest example of how the sequester is damaging the economy and hurting families across the country."

Some travel groups have warned that the disruptions could hurt the economy.

"If these disruptions unfold as predicted, business travelers will stay home, severely impacting not only the travel industry but the economy overall," the Global Business Travel Association warned the head of the FAA in a letter Friday.

Deborah Seymour was one of the first fliers to face the headaches. She was supposed to fly Sunday night from Los Angles to Tucson, Ariz. First her 9:55 p.m. flight was delayed for four hours. Then at 2 a.m., Southwest Airlines canceled it.

"It's pretty discouraging that Congress can't get it together, and now it's reached the point that we can't get on an airplane and fly," Seymour said.

On some routes Monday, it was actually faster to take ground transportation. The 8 a.m. US Airways shuttle from Washington to New York pushed back from the gate six minutes early but didn't take off until almost 10 a.m.

The plane landed at 10:48 a.m. - more than two and a half hours late. If travelers instead took Amtrak's 8 a.m. Acela Express train from Washington, they arrived in New York at 10:42 a.m. - four minutes early.

Normally, there are 10 air traffic controllers at a regional facility handling arrivals for Los Angeles International Airport. On Sunday night, there were just seven, according to Mike Foote, a local union president with the National Air Traffic Controllers Association. A low layer of clouds late compounded the situation.

In such weather, two controllers do nothing but watch planes as they descend below 15,000 feet to ensure they don't veer off course. That allows 68 to 70 planes to land each hour. Because of the furloughs, there were no controllers to do that Sunday, dropping the arrival rate to 42 planes an hour, Foote said.

United Airlines said there were "alarming pockets" of delays and warned that if a solution isn't found, the problem could "affect air travel reliability for our customers."

Delta Air Lines cautioned travelers to expect delays in New York, Philadelphia, Fort Lauderdale, Fla., Chicago, San Francisco, Los Angeles and San Diego.

Many flights heading to Florida were seeing delays of up to an hour. By late Monday, delays into Los Angeles were expected to average three hours.

Having just one fewer controller to handle arrivals to Newark Liberty International Airport can result in the airport being unable to use a relief runway to handle peak traffic, reducing arrivals by about 15 percent, said Dean Iacopelli, a union official at an FAA regional facility for New York's airports.

"It is not just telling one out of 10 people to stay home and so one out of 10 planes get delayed. It's much more complicated that," Iacopelli said.

Prior to the furloughs, if a controller called in sick, there were enough people to take on the extra work, Iacopelli said, or somebody could be asked to work overtime. Now that isn't possible.

The FAA has also furloughed other critical employees, including airline and airport safety inspectors.

In a letter to the FAA Friday, Delta general counsel Ben Hirst asked the agency to reconsider the furloughs, saying it could make the cuts elsewhere and transfer funds from "non-safety activities" to support the FAA's "core mission of efficiently managing the nation's airspace."

Two airline trade associations and the nation's largest pilots union filed a lawsuit Friday asking the U.S. Court of Appeals to halt the furloughs. No hearing date has been set.

The two airline associations - Airlines for America, which represents major carriers, and the Regional Airline Association - are asking the court to place a moratorium on enforcement of the Department of Transportation's three-hour limit on the length of time airlines can keep passengers waiting inside planes on the tarmac without giving them the opportunity to return to a terminal.

Airlines can be fined as much as $27,500 per passenger for violating the limit. The Transportation Department said it is reviewing the industry's request.

Associated Press writers Joan Lowy in Washington and Christopher Weber in Los Angeles contributed to this report.

Scott Mayerowitz can be reached at http://twitter.com/GlobeTrotScott .