WASHINGTON (Reuters) - Two more air traffic controllers were suspended this week for sleeping on the job, including one in Nevada on Wednesday while a medical flight was trying to land, officials said.

The latest incident in Reno, 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 Federal Aviation Administration 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 fell asleep and was out of communication for 16 minutes while a medical flight carrying a patient was trying to land at about 2 a.m. on Wednesday, officials said.

The pilot was in contact with another FAA facility and landed safely.

Heidi Jared, a spokeswoman for the Reno-Tahoe Airport Authority, said the head of her facility immediately called federal officials on Wednesday, to demand action after the controller was found to have fallen asleep.

The airport does not have scheduled commercial flights from midnight to 5 a.m., but it does have military, cargo and medical flights during that time.

"We have a beautiful state-of-the-art, brand new air traffic control tower opened in the fall of 2010, but if that tower is not staffed, we're not making use of the equipment that's now in place," she said.

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 in Washington and Alex Dobuzinskis in Los Angeles)