An A-10 Thunderbolt II flies over U.S. soldier during training at the Yukon Training Area, Alaska, on Aug. 20, 2014. (Photo by Justin Connaher/ U.S. Air Force)

The Air Force has begun moving A-10 attack jets into backup status, even as it continues to use the plane in airstrikes against the Islamic State.

Air Force officials announced the move on Friday. Eighteen planes will be placed into “Backup-Aircraft Inventory” (BAI) status, with the possibility of 18 more being mothballed later this year. The designation means the Thunderbolt II planes are considered to be undergoing maintenance or otherwise unable to fly.

The decision already is under fire by some members of Congress, who are frustrated by the Air Force’s plan to retire the plane, nicknamed the Warthog. Eight Republican senators sent a letter to new Defense Secretary Ashton B. Carter, saying that moving A-10s to backup status now is effectively a “‘back-door divestment” of the plane, even though Congress voted to stop the Air Force from retiring the plane in 2015.

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“The A-10 is the Air Force’s most combat-effective and cost-efficient close air support aircraft,” said the senators. “Close air support experts believe that the A-10 provides capabilities that no other aircraft can replicate. This is not just another fight over a Department of Defense weapons program; this is about what kind of help we will provide our ground troops when they are pinned down by enemy fire and call for help.”

The senators who signed the letter are Kelly Ayotte (R-N.H.), Tom Cotton (R-Ark.), Lindsey Graham (R-S.C.), Thom Tillis (R-N.C.), Roger Wicker (R-Miss.), Mike Crapo (R-Idaho), Johnny Isakson (R-Ga.), and Richard Burr (R-N.C.).

Air Force Secretary Deborah James said in a news release that the new National Defense Authorization Act gives the Pentagon the authority to move 36 A-10 jets into backup status, but officials are electing to move only 18 now “out of respect for the the intent of Congress.” The service is still assessing what to do with the other planes.

“We will revisit this action as the year progresses to assess the need to put the additional 18 aircraft into [backup] status,” said Air Force Chief of Staff Gen. Mark Welsh III. “This action represents the difficult choices required to balance between maintaining the capacity to meet current operational requirements and the resource investment required to keep our modernization efforts on schedule.”

The slow-moving jet isn’t designed for air-to-air combat with other planes, but is beloved by ground troops for its ability to deliver close-air support. It has an armored belly to protect pilots from ground fire, and is armed with a 30mm Gatling gun cannon and a variety of bombs, missiles and rockets. It has been in the Air Force since the 1970s and has flown in combat missions since the 1990s.

A U.S. airman works on one of several A-10 Thunderbolts II that arrived at an undisclosed base in southwest Asia on Nov. 17 to assist in the fight against the Islamic State militant group. (Photo by Tech. Sgt. Jared Marquis/ U.S. Air Force)

The Air Force intends to retire the A-10 and use newer planes for close-air support to save money. Senior defense officials have said that getting rid of the Air Force’s 283 A-10s will save $3.7 billion over five years, but the move has been decried by critics who see it as uniquely suited to help U.S. ground troops in combat.

The 18 A-10s the Air Force wants to put in backup status now are split between three bases. They are Davis-Monthan Air Force Base in Arizona, Moody Air Force Base in Georgia, and Nellis Air Force Base in Nevada. Nine planes will be mothballed at Davis-Monthan, with six put in backup status at Moody and three at Nellis.

The Air Force’s budget request for fiscal 2016 calls for the service to retire 164 A-10s, with all of them gone by 2019.