Image caption Concerns grew after two planes had to land without help in Washington because of a sleeping controller

The head of the US air traffic control agency has resigned after a number of incidents where air traffic controllers fell asleep while on duty.

Randy Babbitt, chief of the Federal Aviation Administration, said he had accepted Hank Krakowski's resignation.

On Thursday, Mr Babbitt pledged a "top to bottom review" of the air traffic control system.

In the past month, several planes have landed safely at US airports without controller guidance.

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

"This conduct must stop immediately."

Mr Babbitt said David Grizzle, chief counsel of the Federal Aviation Administration (FAA), would assume Mr Krakowski's duties as acting head of the Air Traffic Organization.

Night shifts

Mr Babbitt announced on Wednesday that the FAA would place an additional air traffic controller on the midnight shift at 27 control towers across the country; previously, they had been staffed with only one controller during that shift.

The issue rose to prominence last month when two jets were forced to land at about midnight at Reagan National Airport, just by Washington DC, without help from the local control tower.

The pilots, who carried 165 people aboard the two planes, were unable to raise the tower controller on the radio, and a subsequent investigation revealed he had inadvertently fallen asleep during the shift.

This week, the FAA revealed that an air traffic controller at a major airport in Seattle had fallen asleep during a morning shift on Monday; the FAA said he had also fallen asleep on two separate occasions during an early evening shift on 6 January.

And a controller in the US state of Nevada was asleep and out of communication for about 16 minutes on Wednesday while a medical plane was landing, federal officials have said.

An air traffic controller in Tennessee was found to have lain down for a nap during an overnight shift in February, the Washington Post reported.