The U.S. Transportation Department said Thursday it is taking steps to retain and recruit more air traffic controllers to address a significant staffing shortage.

The Federal Aviation Administration, which said earlier it plans to hire 2,000 air traffic controller trainees this year, said it will offer retirement-eligible controllers who are under the mandatory retirement age of 56 a lump sum payment of 20% of their basic pay for each year they continue to work.

A persistent shortage of controllers has delayed flights and many controllers are working mandatory overtime and six-day weeks. The FAA is about 3,500 air traffic controllers short of targeted staffing levels.