Striking mechanics at Northwest Airlines could lose their jobs to replacement workers, the airline threatened in a letter to the union.

NEW YORK (CNN/Money) - Northwest Airlines could meet with its striking mechanics union Thursday, but it warned that it is getting ready to permanently hire replacement workers it has used during the nearly three-week long job action.

The nation's No. 4 airline said it has tentative plans to resume negotiations on Thursday with Aircraft Mechanics Fraternal Association, which represents 4,400 striking mechanics and other maintenance workers. The talks would be the first since the start of the strike Aug. 20.

But the airline says it is now looking at even deeper labor cost cuts than those rejected by the union before the strike, and that it will start hiring replacement workers on a permanent basis on Tuesday if it doesn't have an agreement with the union by then. The carrier said that work previously done by union members, cleaning aircraft and performing maintenance outside of Northwest's main hubs, has already been permanently shifted to outside contractors.

Shares of Northwest (Research) gained 17 cents, or five percent, to $3.54 in pre-market trading on Inet. Shares are still down by more than a third since the day before the strike, though that decline followed a spike in jet fuel prices that the airline has warned would increase the risk of a bankruptcy filing.

During the strike, the airline has continued flying using about 1,200 replacement workers, members of management and outside contractors. Unions at Northwest representing pilots, flight attendants and other ground workers have stayed on the job through the AMFA strike.

A spokesman for the union declined to confirm that a meeting would take place. He said the union is awaiting word from the National Mediation Board, the government body that oversees labor talks in the airline industry.

"We presume there is going to be (a meeting)," AMFA spokesman Jeff Mathews said. He added that news from the NMB could come any time.

In a letter to AMFA dated Sept. 6 that Northwest released to the media, Northwest Vice President of Labor Relations Julie Hagen Showers said the current operation is by nature an "interim measure.

"Given the rapidly changing environment in which we must compete, the company now needs to adopt a permanent solution for this segment of its workforce in a long-term effort to secure the future of the airline and its employees," said her letter.

The letter also invited striking AMFA members to return to work to fill open positions or jobs now held by replacement workers, but said that the company would start filling mechanics' jobs on a permanent basis on Sept. 13.

Northwest said it was willing to resume talks with AMFA, but that due to skyrocketing fuel prices it can no longer stand behind its proposal made just ahead of the strike. At that time, it was seeking to cut top pay for mechanics by 25 percent to $27.28 an hour from $36.39. The union charges that other demands from management would lead to about half of its members at Northwest losing their jobs.

Northwest has said before the strike and the recent jet fuel price jump that it needed $1.1 billion in annual labor cost savings to restructure and avert bankruptcy. The carrier was seeking $176 million of that amount from mechanics. But on Friday it warned it is now looking at a third quarter loss of between $350 to $400 million, excluding special items. And it warned Friday in an Securities and Exchange Commission filing and in the letter to AMFA that the recent leap in jet fuel prices had reduced the time it had to win concessions before it would be forced to file for bankruptcy court protections, although it did not give a specific deadline.

"If AMFA desires to return to the negotiating table, the company is certainly willing to do so," Showers wrote. "The company remains willing to reach a consensual agreement with AMFA and thereby end the strike."

Late Wednesday, Northwest Airlines also said it would suspend indefinitely its daily nonstop service from New York's John F. Kennedy airport to Tokyo's Narita airport beginning Oct. 2, due to fuel cost pressures.

"Unfortunately, the record-high cost of fuel, a challenge facing the entire airline industry, is severely affecting the financial viability of some routes and forcing us to make some difficult decisions," said Phil Haan, a Northwest vice president.

For a look at the implications of Northwest's use of replacement workers for the broader labor movement, click here.