FAA said it will begin closing 149 contract air traffic control towers April 7 to comply with mandatory budget cuts.

The towers scheduled for closure are run by certified FAA contractors at smaller US airports that mostly handle general and business aviation traffic, though a small percentage of commercial flights will be affected. FAA pointed out that it scaled back from its original proposal to close 189 contract towers to comply with budget sequestration, which mandates more than $620 million in agency spending be reduced over the next six months.

“We heard from communities across the country about the importance of their towers and these were very tough decisions,” US transportation secretary Ray LaHood said in a statement.

“Unfortunately, we are faced with a series of difficult choices that we have to make to reach the required cuts under sequestration.”

The closures will occur on a phased basis over a four-week period. To comply with the sequestration cuts, FAA also plans to begin furloughing most of its employees—including air traffic controllers—for one day every two weeks starting next month, which it has warned will lead to flight delays.