The U.S. Department of Defense has grounded all F-35 jets again after a string of accidents, including the latest in which one of the radar-evading aircraft caught fire as it was preparing to take off on June 23.

The temporary grounding has triggered concerns in South Korea over a possible delay in the delivery of the warplanes. Seoul plans to purchase 40 F-35 jets for deployment from 2018-21.

Seoul has requested that the U.S. government provide an explanation on the latest accident.

“We have asked the U.S. to explain what exactly happened. For now, we are just waiting for the results of the investigation into the cause of the latest accident,” said Baek Youn-hyeong, spokesperson of the Defense Acquisition Program Administration.

It’s the eighth time the entire F-35 fleet has been grounded. On June 13, an engine oil leak also led the U.S. military to inspect 104 F-35 jets that have been produced since 2006.

Lockheed Martin, the manufacturer of the F-35, said it is working closely with the ongoing investigation, arguing that safety is its top priority. The U.S. government’s F-35 Joint Program Office vowed to thoroughly look into the recent accident.

“We will contribute to the return to flight determination and will aim to do what is prudent for the enterprise at large without compromising the ongoing mishap investigation,” the JPO said in a statement.

Controversy has lingered over the F-35 development program due to the discovery of defects, development delays and surging costs. The program, the largest-ever defense program for the U.S., is already seven years behind schedule. The Pentagon is to spend some $400 billion to secure the 2,443 planes.

Earlier this year, South Korea confirmed its plan to purchase 40 F-35s with a budget of 7.4 trillion won ($7.3 billion). The F-35 was the sole candidate in Seoul’s next-generation fighter procurement project as it was the only all-stealth airplane available on the market.

Seoul plans to purchase the F-35A, which is the fighter’s Air Force variant. The F-35B is the Marine Corps’ short takeoff and vertical landing variant and the F-35C the Navy’s carrier-based version.

The F-35 was developed under the F-35 “Joint Strike Fighter” project launched in 2001. The project is a multinational program including the U.S. and eight foreign partners: Britain, Italy, the Netherlands, Turkey, Canada, Australia, Denmark and Norway.

By Song Sang-ho (sshluck@heraldcorp.com)