The South Korean Air Force announced Thursday that the United States and South Korea conducted their first joint exercise with F-35A stealth fighter jets considering North Korea’s impending nuclear test.

The four-day training exercise began on Monday and involved 30 aircraft, including South Korean F-35A, F-15K, KF-16, and FA-50 aircraft, as well as U.S. F-35A and F-16 jets fighter.

“These exercises are arranged to improve the performance of joint operations through practical training and to improve the interoperability of the F-35A, the fifth-generation fighter jet jointly operated by the two countries,” the ROK Air Force statement said.

U.S. F-35A pilot Lt. Col. Ryan Worrell said this week’s training was to “ensure the security of the Korean peninsula and strengthen the Korea-U.S. alliance.”

So far, Seoul has purchased 40 fifth-generation fighter jets built by Lockheed Martin and plans to buy 20 more in the next few years.

Last week, the United States dispatched six F-35A fighter jets to South Korea from Eielson Air Force Base in Alaska. The deployment of stealth fighter jets to the Korean peninsula for the first time since December 2017 follows an agreement between Seoul and Washington to intensify joint military exercises.

Pyongyang has conducted at least 18 weapons tests in 2022, including the first launch of an intercontinental ballistic missile in 2017. Officials in Seoul and Washington have said in recent weeks that North Korea appears ready to conduct a seventh nuclear test anytime.