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 expand their partnership "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 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 panel's ranking Republican.

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