Drone aircraft recently carried out unauthorized intrusions over Air Force and Navy nuclear facilities, and the incidents pose a growing threat, the commander of the U.S. Strategic Command disclosed to Congress Wednesday.

Gen. John E. Hyten revealed the drone threats in written testimony before the House Armed Services Committee for a hearing on nuclear deterrence.

“Of recent concern have been the unauthorized flights of unmanned aerial systems (UAS) over Navy and Air Force installations,” Gen. Hyten said. “These intrusions represent a growing threat to the safety and security of nuclear weapons and personnel.”

Currently, the Navy and Air Force are planning to deploy counter-unmanned aerial system defenses that Gen. Hyten said will “effectively detect, track and, if necessary, engage small UAS vehicles.”

The commander said protecting U.S. nuclear forces and facilities is a top priority.

“We are continually assessing threats to ensure our security apparatus is capable of denying unauthorized access or use of nuclear weapons,” he said.

No other details were disclosed about the location of the incidents, or how many took place.

Current Air Force and Navy nuclear facilities include Barksdale Air Force Base, Louisiana; Whiteman Air Force Base, Missouri; Bremerton Naval Submarine Base, Washington state; Kings Bay Naval Submarine Base, Georgia; Malmstrom Air Force Base, Montana; Nellis Air Force Base, Nevada; Minot Air Force Base, North Dakota; and F.E. Warren Air Force Base, Wyoming. The bases are home to U.S. land-based missiles, bombers and nuclear missile submarines.

There have been no news reports of drones making flights over nuclear facilities.

Deputy Defense Secretary Bob Work warned during a visit to the Kings Bay naval base in April that small drones could penetrate security at nuclear missile and submarine bases. He mentioned that a Netherlands company, Guard From Above, uses birds to attack drones that fly near nuclear power plants, Defense News reported.

The Air Force Global Strike Command, which handles security at nuclear sites, is purchasing electronic equipment capable of disrupting the signals that control drones.

The most recent National Defense Authorization Act, signed in December, contains a provision that calls on the Energy Department to protect nuclear sites from threats posed by unmanned aircraft. The Energy Department operates 10 nuclear sites in the United States.

Drones were detected flying over the Savannah River Site nuclear arms complex last year, and FBI agents questioned an anti-nuclear activist about the flights, the Columbia, South Carolina, newspaper The State reported.

The Navy in February 2016 investigated a drone that was spotted flying over Naval Base Kitsap-Bangor, where U.S. nuclear missile submarines are deployed.

Gen. Hyten and other senior military leaders in charge of nuclear weapons warned during the hearing that any delays in carrying out an urgently needed nuclear weapons modernization program would increase the risk of undermining that U.S. deterrence of nuclear adversaries.

“Nuclear modernization can no longer be deferred,” said Gen. Hyten. “Any disruption of the current program of record for future acquisition plans will introduce the risk — significant risk to our deterrent.”

The military is building new missiles, missile submarines and bombers to replace currently aging systems.

U.S. nuclear cuts during the Obama administration were not matched by China and Russia, and both countries are rapidly modernizing their nuclear forces, the military leaders testified.