Air traffic control in the United States is understaffed by about 3,000 positions, Transportation Secretary Pete Buttigieg told CNN on Friday, as hiring to fill some of that gap opened.

The Federal Aviation Administration ATC workforce currently numbers about 11,500 controllers, but the optimal number is closer to about 14,500, the secretary said on CNN News Central. The numbers are similar to the agency's assessment this spring that about one in five controller positions nationwide are vacant.

We have thousands of controllers in training right now but also a number who are eligible to retire, Buttigieg said.

He spoke as the agency began hiring on Friday -- targeting 1,500 new entry-level air traffic controllers this year -- and published an annual report on the agency's ATC workforce.

That report showed the agency hired 1,026 controllers last year, just shy of its targets, acting FAA administrator Billy Nolen told Congress in a letter. The FAA also experienced more controllers leaving the job last year than it had planned, and is further short-staffed because the coronavirus pandemic has resulted in delayed certification for most existing developmental controllers.

Recently, Nolen told reporters that the agency's planned 3,300 hires in the next two years will mostly replace those who are retiring. About 500 of those hires will hold positions that are currently empty.

Earlier this spring, the FAA asked airlines to dial back flights this summer in the New York metropolitan area, where a key radar facility is only 54 percent staffed. Airlines flying their planned summer schedules would cause a spike in delays due to the shortage, the FAA predicted.

But Buttigieg argued that most delays travelers experience are not the cause of air traffic controller short-staffing.

Controller availability is not the cause of most cancelations and delays that we see, he said in the CNN interview. The (staffing) gaps that we've seen have built up over years. This is nothing that we can't prepare for going into this summer. But this is part of why we're working so hard to train new air traffic controllers.

Nolen wrote in a letter to Congress that about 5 percent of delay minutes can be attributed to FAA staff shortages.