WASHINGTON - The US official overseeing air traffic operations resigned on Thursday over a controversy involving disclosures about sleeping air traffic controllers.

Hank Krakowski, who is director of the FAA's Air Traffic Organization, stepped down amid the revelations, the agency said in a statement.

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Krakowski's resignation was accepted by FAA Administrator Randy Babbitt, who said he was conducting a full review of air traffic operations.

"Over the last few weeks, we have seen examples of unprofessional conduct on the part of a few individuals that have rightly caused the traveling public to question our ability to ensure their safety," Babbitt said in a statement. "This conduct must stop immediately."

US aviation regulators suspended two air traffic controllers this week for falling asleep on the job, including one who was out of radio contact while a medical flight was trying to land.

The latest incident early on Wednesday in Nevada and one on Monday at Boeing Field in Seattle added to concerns about safety prompted by two other controllers found to have slept at work in recent months in Washington, D.C., and Tennessee.

The incidents have raised questions about adequate overnight staffing at air traffic towers. The FAA said in response to Wednesday's incident at the Reno-Tahoe International Airport that it would add controllers at 27 towers.