BRUSSELS—The U.S. is preparing to deliver its first formal objections to the European Union's impending emission-trading plan for airlines, said people familiar with Washington's position, ratcheting up global pressure on the EU to scale back its ambitions.

China, Russia and other major countries, as well as airlines world-wide, have recently criticized the project. The pollution-control plan, which is set to include aviation starting in January, forces any carrier departing or arriving at an EU airport to buy credits for...