The Army has grounded its entire fleet of CH-47 Chinook helicopters while it works to fix fuel leaks that caused an unspecified number of engine fires, the service confirmed.

In a statement, the Army said it was grounding the fleet of about 400 aircraft “out of an abundance of caution,” stressing that no deaths or injuries resulted from the fires.

“The Army has identified the root cause of fuel leaks that caused a small number of engine fires among an isolated number of H-47 helicopters and is implementing corrective measures to resolve this issue,” Army spokesperson Cynthia Smith said in an email. “The safety of our soldiers is the Army’s top priority, and we will ensure our aircraft remain safe and airworthy.”

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The statement did not provide details on the repair timeline, nor what operations or training could be affected by the grounding, which was first reported by the Wall Street Journal.

But the statement did suggest that some Chinooks could be back in the air shortly. About 70 helicopters are believed to have a faulty part that’s been connected to the problem, according to the Journal.

“Based on the results of our investigation, some aircraft may not require corrective measures and may soon return to normal flight operations,” Smith said.

The Chinook has been a staple of the Army fleet since its introduction 60 years ago, carrying troops and cargo on battlefields from Vietnam to Afghanistan and Iraq. The current model, the CH-47F, is the Army’s “only heavy-lift cargo helicopter supporting combat and other critical operations,” according to the service’s website.

The issue that prompted the grounding stems from an engine part known as an O-ring, according to a statement from Honeywell International, which manufactures the engine of the Boeing-made Chinook. O-rings are used to create a seal between engine parts to prevent leaks.

While investigating incidents involving the Chinooks, Honeywell and the Army discovered O-rings that were “not meeting Honeywell design specifications” and were installed in the engines during routine maintenance, according to the contractor. The faulty O-rings weren’t made by Honeywell, the statement added.

“It is believed these suspect O-Rings have been identified and isolated,” Honeywell said. “Joint U.S. Army and Honeywell engineers identified the issue, and are now working with the Army to provide replacement O-rings on all affected Chinooks.”

The grounding of the Chinook fleet is the latest in a string of major aircraft issues for the military. Earlier this month, Air Force Special Operations Command announced it was grounding its entire fleet of CV-22 Osprey tiltrotor aircraft over an issue with the clutch. The Marine Corps, meanwhile, said it would continue to fly its Ospreys since the service has known about the clutch issue since 2010 and has trained its pilots to work around the problem.

Ejection seat issues also caused the Air Force, Navy and Marine Corps to ground hundreds of aircraft in July, including F-35A Lightning IIs, F/A-18 Hornets, F/A-18E/F Super Hornets, EA-18G Growlers, as well as T-45 Goshawk, F-5 Tiger II, T-38 Talon and T-6 Texan II training aircraft.