ATLANTA -- Delta Air Lines pilots have been granted permits to picket at Northwest Airlines hub airports amid a dispute over seniority between the two airlines' unions that so far has held up a merger of the carriers.

A person familiar with the situation said on Tuesday that the permits grant Delta's pilots permission to picket at the Minneapolis and Detroit airports, which are considered Northwest crew bases, from Thursday to April 24.

Northwest's pilots union told its members in the memo that Delta's pilots have been granted such approvals in the past but have not followed through and actually picketed. The union said it reserves the right to do the same thing at Delta hub airports if it chooses.

A spokeswoman for Delta's pilots union did not respond to an e-mail Tuesday night seeking comment.

People familiar with the merger discussions said earlier this week that Delta is trying to determine whether to push ahead with a combination with Northwest without a prearranged deal between their pilots, a move that could change details of the deal.