The US Air Force has temporarily grounded dozens of F-35 stealth fighters on one of its bases while it investigates an apparent oxygen supply issue aboard the expensive planes, officials said.

The Israeli Air Force has not taken similar action with its five F-35 fighter jets, but is in contact with the aircraft’s developers and monitoring the situation, an IDF spokesperson said Tuesday.

The American planes, from the USAF’s 56th Fighter Wing of Luke Air Force Base in Arizona, were grounded Friday due to five incidents since May 2 in which pilots experienced hypoxia-like symptoms. It is the only American F-35 base to take such a measure.

USAF spokeswoman Major Rebecca Heyse said in a statement Monday that the planes “will continue their pause in local F-35A flying to coordinate analysis and communication between pilots, maintainers, medical professionals and a team of military and industry experts.”

Israel's first two F-35 stealth fighter jets on their maiden flight as part of the Israeli Air Force on December 13, 2016. (IDF Spokesperson's Unit)

Israel’s first two F-35 stealth fighter jets on their maiden flight as part of the Israeli Air Force on December 13, 2016. (IDF Spokesperson’s Unit)

Currently, the 56th Fighter Wing has 55 F-35A planes that are used to train pilots from the US and other air forces that are buying the jet.

Heyse said it was unclear when the “pause” would be lifted, but noted that none of the incidents were serious and pilots were able to use their training to avert any problems.

“The safety of our airmen is paramount and we will take as much time as necessary to ensure their safety,” she said.

Another spokesman, Captain Mark Graff, said in a statement that a backup oxygen system kicked in as designed.

The American planes that experienced this oxygen supply issue are of the “A” variety of the aircraft. Israel operates the “I” model, though it is based on the design of the “A.”

Despite being the same general model, the IAF has not experienced similar oxygen supply problems with the F-35 jets at its disposal, nor did any of the other US Air Force bases that operate the F-35A.

Therefore, the IDF spokesperson said, “there has been no decision to cancel training exercises or to take any other steps.”

The F-35 has been dogged by criticism nearly from its inception, with complaints over its high costs and production issues, though in recent years much of the vitriol surrounding the aircraft has subsided, as the development stuck to its schedule and budget.

With a current development and acquisition price tag already at $379 billion for a total of 2,443 F-35 aircraft — most destined for the US Air Force — the F-35 is the most expensive plane in history, and costs are set rise further still. Once servicing and maintenance costs for the F-35 are factored in over the aircraft’s lifespan through 2070, overall program costs are expected to rise to $1.5 trillion.

Proponents tout the F-35’s radar-dodging stealth technology, supersonic speeds, close air-support capabilities, airborne agility and a massive array of sensors giving pilots unparalleled access to information.