The union representing pilots at United Airlines said on Wednesday a merger with US Airways should be a last resort.
The Air Line Pilots Association said in a statement it had "serious concerns" that a merger would not yield the financial benefits some industry experts predict.
United and US Airways are in advanced merger discussions, according to sources with knowledge of the matter, although neither carrier has confirmed the talks.
Many airline experts believe consolidation is the best way to stabilize an industry battered by high fuel costs and low-fare competition.
But the pilots' union said that "the highly-touted financial benefits to be derived from such a merger are unlikely to be achieved because these benefits are based on assumptions that have no basis in reality."
The union said the financial benefits of a merger are dubious, given the labor unrest at US Airways, which merged with America West in 2005 and has yet to integrate its labor groups.
Further, a deal is unlikely to receive approval from the US Justice Department, the union said.