Story highlights Transportation secretary calls it "difficult choices that we have to make"

FAA administrator says agency with work with affected airports to ensure safety

Controllers' group says closures will hurt "overall margin of safety for our entire aviation system"

Among towers are two built with federal stimulus money and open less than a year

The Federal Aviation Administration on Friday told 149 regional airports across the country it would begin closing their air traffic control towers in April, but said it would spare another 40 towers that had been on the chopping block.

The agency said the cuts are necessary to help meet $637 million in forced spending cuts.

The tower closures will begin April 7 and will be phased in over four weeks, FAA Chief Operating Officer David Grizzle said in a memo obtained by CNN. Some towers were spared after the FAA considered appeals from towers, and consulted with the departments of defense and homeland security.

Among those to be closed are towers at regional airports in Frederick, Maryland, and near St. Petersburg, Florida, that were built with federal stimulus money and have been open less than a year.

The decision "will have both short-term and long-term effects," warned the National Air Traffic Controllers Association.

"These towers serve other important functions -- including law enforcement activity, medical transport flights, search and rescue missions, business and commerce and supporting flight schools across America," the association said in a statement.

"Ultimately, the partisan posturing in Washington that led to sequestration is the reason for (Friday's) decision and its destructive effects on aviation."

The FAA had been expected to announce the closure of 189 low- or moderate-volume towers staffed by contractors. Before Friday's announcement, it said it would consider keeping a tower open if the airport convinces the agency it is in the "national interest" to do so.

A news release from the FAA quoted Transportation Secretary Ray LaHood as saying, "We heard from communities across the country about the importance of their towers and these were very tough decisions ... Unfortunately we are faced with a series of difficult choices that we have to make to reach the required cuts under sequestration."

By congressional mandate, the FAA must cut nearly $600 million from its nearly $48 billion budget this fiscal year. Because the majority of its 47,000 employees are air traffic controllers, it is impossible to cut its budget without affecting controllers, the agency said.

FAA Administrator Michael Huerta said in the agency's news release that the agency "will work with the airports and the operators to ensure the procedures are in place to maintain the high level of safety at non-towered airports."

Two Republican members of Congress -- Sen. John Thune of South Dakota, ranking member of the Senate Committee on Commerce, Science and Transportation; and Rep. Bill Shuster of Pennsylvania, chairman of the House Committee on Transportation and Infrastructure -- wrote to LaHood calling for "a detailed justification and explanation for how the FAA determined each contract tower would be closed."

"It is worth noting that, the specified towers in the FAA Contract Tower Program were all operational in 2009, when the FAA received less funding than it will under sequestration," the two said in their letter.

Their was no immediate response from LaHood or the FAA to the letter.