NEW YORK (AP) _ Trans World Airlines, having canceled half its flights because of a strike by flight attendants, has vowed to resume normal operations by midweek but acknowledged the walkout could cost $50 million by then.

The strike by the 6,000-member Independent Federation of Flight Attendants entered its second day today as TWA and union leaders looked to the airline's 10,000 machinists as a key to their showdown.

The strike, which began when the union rejected demands for concessions on wages and working hours, forced the airline to cancel half its flights Friday, TWA Chairman Carl Icahn said.

''Contrary to what you might have heard, all of our employee groups are supporting us,'' Icahn told a news conference in Washington. ''Many of them were up all night and they are working around the clock.''

However, only about 100 machinists reported for work Friday morning at TWA's major maintenance plant in Kansas City, Mo., where 3,200 members of the International Association of Machinists repair the airline's fleet.

''We have qualified management personnel who have come up through the ranks who are qualified to work on the aircraft,'' said TWA spokesman Vic Segale in Kansas City. ''We expect to operate the airline in the next couple of days at 100 percent of our schedule.''

But Ken Weber, treasurer of IAM Local 1650, said he didn't think any of the 1,000 machinists reported for work Friday night at the plant.

''I know I wouldn't want to ride one of the airplanes right now,'' he said. ''I know all the people trained to do that work and they're not there.''

Wayne Clover, president of Local 949 in St. Louis, said he hoped the 1,200 machinists in his local that work for TWA would honor the picket lines.

All 250 TWA machinists in Boston were honoring the picket line, said Don McLaughlin, a committee member of Local 1726.

TWA canceled all service Friday from Newark, N.J., and flights to Copenhagen, Barcelona and Tel Aviv. Today, six of 10 flights to and from London were canceled, according to the TWA check-in counter at Heathrow Airport. The canceled flights were those to and from Chicago, Boston and one of two daily round-trips to New York. Another New York round-trip and flights to and from Los Angeles remained scheduled.

On Friday, the airline also canceled half the 191 daily flights from St. Louis, creating long lines and confusion at Lambert International Airport, where two pickets were arrested.

Ozark Air Lines, a St. Louis-based carrier that recently agreed to a merger with TWA, was accepting all TWA tickets in the same class of service, on a space-available basis.

Ozark, which operates 144 daily flights from St. Louis, said its planes were leaving with empty seats, indicating that TWA passengers were not being stranded.

''When you've done all you can, you just have to sit back and be patient,'' said David Schlessinger of St. Louis, who was trying to get to Europe to attend a science conference. Schlessinger was in a line of 100 people waiting for boarding passes for TWA Flight 94 to New York.

The arrests were made after the two pickets refused to stop picketing a parking lot for airport employees. Lambert airport officials said the lot is used by people other than TWA personnel. Later, a U.S. district judge ordered the airport to ease its picketing rules because of the arrests, pending a hearing Monday.

Icahn said Friday that the airline hoped to restore normal operations within five days, using 1,500 newly hired flight attendants and as many ticket agents and other employees trained for cabin duties. But he estimated that TWA would lose as much as $50 million even if it restores full service as quickly as he hopes.

''The board of directors and management of TWA finally stood up and did something that we had to do,'' Icahn said. ''In the first quarter we're losing $125 million. We cannot exist when we are paying our flight attendants close to $50,000 a year with perks. The average salary is $35,000 per year, and we pay them this to work 11 days a month.''

TWA, under pressure from low-cost competitors and declining traffic, reported losses of $193.1 million last year.

Icahn won concessions earlier this year from the company's 5,000 pilots, who signed a no-strike contract and gave up 34 percent of their wages and benefits, and from the machinists, who gave up 15 percent.

Victoria Frankovich, president of the flight attendants's union, said Friday that no agreement is possible while Icahn demands more concessions from the mostly female members of her union than he extracted from the other, mostly male unions.

Frankovich said Icahn has demanded that her union give back 44 percent in wages and benefits. She said the union offered a 15 percent pay cut, and raised that to 25 percent, but balked at new work rules that would have required two more hours in the air each week for flight attendants.

Icahn said that in the final hours before the talks broke off, he had cut his demands for wage givebacks from 22 percent to 17 percent but was still insisting on the work rule changes.