Delta Air Lines said it has "not arrived at a potential transaction" that meets its goals, the airline told employees in a memorandum Tuesday.

The Atlanta airline did not specifically mention its widely reported merger talks with Northwest Airlines in its message to employees Tuesday afternoon.

However, the memo, titled "Update on Consolidation," seemed to be signaling that the deal has hit a roadblock. The memo could also be a warning shot to the two airlines' pilots unions to reach an agreement on union-integration issues that have threatened to scuttle the deal, which was aimed at creating the largest airline in the world. The merger talks have in effect been stalled while Delta's and Northwest's pilots unions tried to work an agreement to blend their seniority lists.

"To date, we have not arrived at a potential transaction that meets all of our principles," said Delta Chief Executive Richard Anderson and President Ed Bastian in the memo. "Rest assured that we will not complete a transaction unless all of these conditions are met. We have a strong standalone plan. We will maintain our attention on executing that plan while we continue to look at strategic alternatives."

A Delta spokesman declined to comment beyond the memo, which could further ratchet down expectations on Wall Street, where Delta and Northwest shares have been drifting lower in recent weeks as little news of progress surfaced.

The two executives listed several conditions that any deal would have to meet, including requirements that the "seniority of our people would be protected" and that it would speed up Delta's expansion of its overseas network.

Those could be some of the conditions that have been tripped up by the stalled pilot talks. Northwest's pilots have reportedly insisted on seniority integration methods that would in effect push many Delta pilots to the bottom of the seniority list, possibly demoting them to lower-paying pilot jobs.

The unions' integration plan was viewed as a way to reduce potential political opposition to the planned merger and to speed up integration of the carriers once the deal was finalized. Pilots were expected to be rewarded with pay raises and an equity stake as part of the merger.

Anderson and Bastian said in the memo that the combined companies would also have to retain the Delta name and remain headquartered in Atlanta and result in "even greater job security along with more career opportunities for our people."