The National Transportation Safety Board has begun a special safety investigation at Chicago's O'Hare Airport following a series of errors by air traffic controllers.

The federal watchdog said 20 operational errors had been made at the busy airport so far this year including three last week. During all of 1987, there were only 12 such errors."The special investigation will examine the facts, conditions and circumstances of each incident and analyze these data to determine specific corrective actions that would reduce or prevent simmilar occurrences," the board said in a memorandum.

The Federal Aviation Administration, the federal agency that regulates the airlines and operates the air traffic control system, said it supported the probe and would send three of its own investigators to assist the safety board.

"This will permit the FAA to take immediate corrective action if any discrepancies are uncovered," FAA chief Allan McArtor said in a statement.

O'Hare is one of the two busiest U.S. airports, running neck and neck with Atlanta's Hartsfield International Airport.

Human error has been a long-time concern at the facility where controllers handle about 1.2 million aircraft a year.

The FAA said it had recently implemented new procedures to enhance efficiency and reduce the potential for controller errors at O'Hare.

The agency also recently assigned eight more controllers to the facility in anticipation of peak summer travel levels.

The safety board investigates transportation accidents and makes recommendations but has no regulatory powers.