(Reuters) - The stealthy F-35 fighter jets made by Lockheed Martin Corp were temporarily grounded at a U.S. Air Force base in Arizona on Friday because of irregularities in pilots’ oxygen supplies, an Air Force spokesman said.

Training flights were scheduled to resume on Monday after a day of safety briefings at the base on Friday, a base spokeswoman said.

Lockheed Martin plans to demonstrate the advanced jet at the Paris Air Show this month.

At Luke Air Force Base northwest of Phoenix, Arizona, the 56th Fighter Wing canceled local flying operations for its F-35A Lightning II aircraft due to five incidents in which pilots experienced symptoms resembling hypoxia, or oxygen deprivation, according to the Air Force spokesman, Captain Mark Graff at the Pentagon.

The Air Force said the incidents occurred from May 2 to Thursday, and that in each case the aircraft’s backup oxygen system worked as designed and the jets were able to land safely. The base typically has 25 training flights each weekday, the base spokeswoman said.

The Pentagon said that it was conducting a comprehensive review of the facts and circumstances surrounding physiological episodes along with industry experts.

A Lockheed representative said the company would help the Air Force address the issue.

The F-35 business accounted for about 37 percent of Lockheed’s total revenue during the last fiscal quarter, which ended on March 30.

During the quarter, Lockheed’s revenue from its aeronautics business increased 8 percent to $4.11 billion, led by higher sales of the F-35.

Lockheed and its main partners, Northrop Grumman Corp, United Technologies Corp’s Pratt & Whitney and BAE Systems Plc have been developing and building F-35s for the U.S. military and 10 allies.

More than 220 operational F-35s have been built and delivered worldwide, and they have collectively flown more than 95,000 flight hours.