The union, which represents about 6,200 pilots, called the strike as a 30-day cooling off period ended at 12:01 A.M. EDT (0401 GMT), after rejecting a four-year contract proposal that Northwest said included a pay raise that would make the pilot's average pay higher than that at the three largest airlines.

The deal also included a promise of no layoffs for five years for pilots on the payroll when the contract was signed, the company said.

A source familiar with the negotiations said the union was still holding out for a 15-per cent pay increase, while St. Paul-based Northwest was offering nine per cent. Outsourcing of routes to smaller commuter airlines was also a sticking point, sources said.

But while Northwest said that a tentative agreement had been reached on its planned alliance with Continental Airlines Inc, a key part of Northwest's business strategy, a union spokesman said that was not true. The spokesman said other issues were also holding up a contract.

The strike affects about 150,000 passengers and 1,700 flights a day, especially at Northwest hub airports in Memphis, Minneapolis/St. Paul and Detroit. Northwest is based in St. Paul, Minnesota.

Those passengers will not be getting any help from president Bill Clinton at this point. The White House said that Clinton would not intervene to stop the strike, as he did almost immediately after pilots at AMR Corp's American Airlines struck last year.

"At this time, the president has decided not to exercise his authority and he urges the parties to stay at the table, work hard and resolve their differences," the White House said in a statement released just after the strike deadline.

But a spokesman for the Air Line Pilots Association in Memphis, one of Northwest's three major US hubs, said pilots have been told to remain available to resume work for the next half day, meaning the strike could be short-lived.

The company and union have both said they are willing to return to the bargaining table, though sources said talks were not expected to resume overnight.

The pilots are seeking to recoup concessions they made in 1993 when the airline was in dire financial straits.

"We did not save this airline in 1993 to have our jobs outsourced in 1998," pilot union spokesman Paul Omodt said, adding that "the salary increase proposal was an insult to the pilots and the working people of the airline."Northwest spokesman Jon Austin said the pilots refused to make any meaningful compromises on their economic demands.

"Unfortunately, our pilots would not temper their short-term self-interest sufficiently to make possible agreement on the handful of issues that still divide us," he said at a news conference.

At airports in Memphis and Minneapolis, where Northwest is by far the dominant carrier, passengers began lining up to exchange tickets after the strike was announced. The terminals had been quiet Friday, with Northwest having cancelled 400 weekend flights earlier this week due to uncertainty about a strike.

Ronald Neal, of Marion, Arkansas, was worried about what the strike would do for his plans for a vacation for his 9 and 12-year old daughters and a 17th anniversary celebration for he and his wife.

"I'm damn worried," Neal said at the Northwest counter at Memphis-Shelby County International Airport. "I've been scheduled to go on vacation on Tuesday morning. I bought my tickets two months ago we were going to Orlando, Fla. to take my two little girls to Disneyworld." He was later put on a Delta Air Lines Inc. flight. Many airlines have said they will try to accommodate Northwest passengers, but seats are limited during the end of the busy summer travel season.

Meanwhile, the Memphis airport has brought in airmattresses, blankets extra diapers and other supplies for stranded travelers, while in Detroit, cots, diapers baby food and free coffee are among the provisions available.

Northwest has also been reserving hotel rooms for weeks in the event of a strike.