WASHINGTON (Reuters) - U.S. aviation regulators have suspended two more air traffic controllers for falling asleep on the job, including one who was out of radio contact while a medical flight was trying land.

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

"We absolutely cannot and will not tolerate sleeping on the job," FAA Administrator Randy Babbitt said.

Sleeping controllers have raised questions about adequate overnight staffing at air traffic towers, some of which operate with only one person handling a light load of flights.

The FAA said in response to Wednesday's incident at the Reno-Tahoe International Airport that it would add controllers at 27 towers.

The Nevada controller working alone was out of communication for 16 minutes while a medical flight carrying a sick patient was trying to land. The pilot was, however, in contact with another FAA facility and landed safely.

On Monday, a controller at Boeing Field/King County International Airport in Seattle was suspended for falling asleep during his morning shift, the FAA said.

The same controller was already facing disciplinary action for falling asleep on another shift this year.

Babbitt told Congress last week the FAA was moving to fire a controller found sleeping on the job at the Knoxville, Tennessee, airport in February.

This followed an admission to investigators by another controller that he had fallen asleep on the job at Ronald Reagan National Airport close to the U.S. capital on March 23. Two jetliners landed without guidance from the tower.

(Reporting by John Crawley; Editing by Laura MacInnis)