A U.S. airliner nearly collided with a drone over Florida earlier this year, a federal official said, a near miss that highlights risks posed by the proliferation of unmanned aircraft in U.S. skies.

A pilot of an American Airlines Group Inc. regional jet told officials that on March 22 he came dangerously close to a "small remotely piloted aircraft" about 2,300 feet above the ground near Tallahassee Regional Airport in Florida, said Jim Williams, head of the unmanned-aircraft office at the Federal Aviation Administration. Mr. Williams disclosed the incident publicly for the first time at a drone conference in San Francisco on Thursday.

American Airlines said that safety is its top priority and that it is investigating the alleged incident.

"The airline pilot said that he thought the [drone] was so close to his jet that he was sure he had collided with it," Mr. Williams said. Inspection of the aircraft later found no damage, he said, but "the risk for a small [drone] to be ingested into a passenger airline engine is very real."

Manned aircraft normally should be at least 1,000 feet apart vertically and several miles apart laterally.