ATLANTA — Delta and Northwest could announce a combination as early as tomorrow that would create the world's biggest airline, but pilot contract issues still loomed and there was no guarantee the deal would move ahead, three people familiar with the talks said yesterday.

The people, who spoke on condition of anonymity because of the sensitivity of the situation, said officials were mobilizing for an announcement provided the boards of the two companies give final approval to a deal.

The boards of both companies have been having ongoing merger-related conversations. They could meet today to discuss moving ahead with a deal, a person familiar with the talks said.

The deal could see strong opposition from Northwest pilots.

It was increasingly likely the two airlines would proceed without a prearranged seniority integration agreement between their pilots unions, so the main issue that remained was whether Delta pilots were willing to make changes to their contract to give Delta more flexibility after a combination, one of the people said.