Delta Air Lines' pilots union confirmed Monday that it has failed to reach an agreement with Northwest Airlines' pilots union on how the groups would join ranks if the companies merged.

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Pilot seniority was the sticking point, as it has been for several weeks of on-again, off-again talks. Seniority determines a pilot's pay, schedule and plane assignments.

"Today, I can report that while much was accomplished during a relatively short period of time, we have been unable to reach an agreement on a seniority list integration," Lee Moak, chairman of the Delta unit of the Air Line Pilots Association, wrote in a letter to members, which was posted on the union's Web site.

"Our intent was to accomplish what has never before been done in our industry -- reach a three-party agreement between [the two unions and company management] in advance of any corporate merger announcement," Moak wrote.

A proposed merger between Delta and Northwest isn't necessarily dead. But Atlanta-based Delta has said getting pilots to agree to how they would merge work loads is key to any deal.

A Delta spokesman declined to comment on ALPA's letter, noting that a committee of the airline's board of directors is still looking at various strategic options for the company.

"Delta's special committee of the board remains active and continues to work with the senior leadership team to review ... potential consolidation transactions," said company spokesman Kent Landers.

Industry analyst Ray Neidl said Delta's merger plans are probably on hold. "I think it proves the fact that pilots can't agree on anything and shouldn't be dictating the future of mergers," said Neidl of Calyon Securities in New York.

Still, he added, it was a "smart thing" for Delta to try to get pilots to work out difficult issues ahead of time, given the troubled history of integrating unions in previous airline mergers.

Moak's letter coincides with cost-cutting changes to the airline's operations. Delta is expected Tuesday to announce what it will do to try to offset pressures of record jet fuel prices, possibly including reductions to its capacity and work force.

Most big airlines' shares have plunged about 40 percent this year, especially in recent weeks as fuel prices soared and the Delta-Northwest deal's prospects withered. The proposed merger would create the world's largest airline. Delta's shares fell 4 percent Monday to $9.23; Northwest's shares fell 6 percent to $8.92. The spot price of jet fuel, both airlines' biggest expense, was $3.24 a gallon, down slightly from Friday's all-time record.

Union negotiators have been working since January. A senior Delta pilot and other people familiar with the talks have said the unions worked out most terms of a pact that would have given Northwest pilots about a 30 percent raise over four years and up to a 7 percent stake in the company. Delta's higher-paid pilots would have received a smaller pay hike, but equal equity.

Moak said the unions' negotiating committees and management reached a tentative agreement in the third week of February on all issues except seniority.

But seniority is especially complicated: Roughly 2,000 of Delta's most senior pilots retired before the carrier filed for bankruptcy in 2005. That means many Delta pilots could have been demoted under the integration scenario favored by Northwest's union, or that Northwest's more senior pilots could have faced diminished promotion prospects under the plan Delta's union favored.

"We presented a rational and fair integration method" aimed at reaching a compromise quickly, Moak wrote. "The other committee took another approach," he said, referring to Northwest's union, also a unit of ALPA.

Union negotiators were reached an impasse last weekend, Moak wrote: The Northwest union's final proposal "would jeopardize the seniority and career expectations of Delta pilots."

A spokesman for the pilots union at Eagan, Minn.-based Northwest, Greg Rizzuto, told the Associated Press on Monday that his union "still values any deal to help better the careers of all pilots involved in any type of future merger or acquisition with any pilot group, and due to the rising cost of oil it is imperative that a fair integration of seniority lists be found between any group."

Delta and Northwest put their negotiations on hold last month to give the pilots' unions time to resolve remaining issues. The airlines wanted such issues worked out ahead of time to avoid delays and employee and political opposition later.

US Airways and AmericaWest did not get pre-merger agreements from their pilots before they combined in 2005. Today, those pilots still are working under separate seniority lists and contracts, weighing down operational integration.