Two leading U.S. senators on antitrust issues said Tuesday that a plan by British Airways PLC (BAY.LN) and AMR Corp.'s (AMR) American Airlines Inc. to intensify cooperation on trans-Atlantic routes "raises significant competition issues," increasing pressure on U.S. regulators reviewing the plan.

Sen. Herb Kohl, D-Wisc., and Sen. Orrin Hatch, R-Utah, expressed their concerns in a letter to U.S. Transportation Secretary Ray LaHood and a Justice Department antitrust official, who are reviewing an application for antitrust immunity that would allow American Airlines, British Airways and Spain's Iberia to expand cooperation within the oneworld alliance.

"While not expressing any views on the ultimate merits of this application, we believe this alliance application raises significant competition issues and we reiterate our views that the DOT act cautiously," the senators wrote. Kohl is the chairman of the Senate Judiciary Committee's subcommittee on antitrust, competition policy and consumer rights, and Hatch is the subcommittee's ranking Republican.

The senators also vowed to "closely examine international aviation alliances...and their likely effects on competition."

The senators are the latest to add their voices to the debate over whether to allow American Airlines and British Airways to work more closely on schedules and prices, and share revenue. The U.S. Transportation Department has said it expects to decide by Oct. 31 on the antitrust-exemption application. The high-profile case is viewed by the industry as a test of evolving U.S. aviation and competition policy.

The European Commission recently issued a so-called statement of objections to the proposed partnership.

Attempts to reach officials at British Airways and American Airlines were unsuccessful. A U.S. Transportation Department spokesman didn't immediately respond to a request for comment.

-By Josh Mitchell, Dow Jones Newswires; 202-862-6637; joshua.mitchell@dowjones.com