Differences between the U.S. Department of Transportation and Department of Justice are contributing to delays in the Transportation Department's ruling on a proposed alliance between AMR Corp.'s (AMR) American Airlines, British Airways PLC (BAY.LN) and Spain's Iberia Lineas Aereas de Espana SA (IBLA.MC), according to people familiar with the case.

The Department of Transportation, or DOT, will not issue a preliminary ruling on the three carriers' application for antitrust immunity by its original Oct. 31 deadline, these people said. The delay will be at least two weeks and could push a final ruling into early next year, these people added.

AMR Chief Executive Gerard Arpey said last week that the DOT may not decide by its Oct. 31 deadline, but he gave no details and said he was optimistic that regulators will approve the airlines' request.

American, BA and Iberia have applied to antitrust authorities in the U.S. and the European Union for permission to cooperate on marketing and strategic planning in ways normally deemed illegal collusion. The trio, which cooperate in the oneworld airline-marketing alliance, argue they should be granted antitrust immunity because their rivals in the Star Alliance and the SkyTeam alliance have already received similar status from the U.S.

By Daniel Michaels, The Wall Street Journal; 0032-2-741-1325; daniel.michaels@wsj.com; and Kaveri Niththyananthan, Dow Jones Newswires; 4420 7842 9299; kaveri.niththyananthan@dowjones.com