U.S. Air Force F-35A stealth jets made their first publicly disclosed trip to South Korea since 2017 on Tuesday as the allies and nuclear-armed North Korea engage in an intensifying cycle of military exercises.

Joint military exercises had been publicly reduced in recent years, first in 2018 as a result of diplomatic efforts to engage North Korea and then later as a result of the COVID-19 crisis.

In order to offset North Korea’s unprecedented number of missile tests this year, South Korean President Yoon Suk-yeol, who took office in May, has sought to expand public displays of the alliance’s military might, including exercises.

North Korea also appears to be preparing to test a nuclear weapon for the first time since 2017.

The six F-35As will be in South Korea for 10 days, South Korea’s Ministry of Defence said in a statement.

“The purpose of this deployment is to demonstrate the strong deterrent and joint defence posture of the U.S.-ROK alliance while at the same time improving the interoperability between the ROK and U.S. Air Force,” the ministry said, referring to South Korea by the initials of its official name.

The aircraft deployed from Eielson Air Force Base in Alaska, U.S. Forces Korea (USFK) said in a statement.

A USFK spokesperson said it was the first public deployment of the 5th generation fighter aircraft to South Korea since December 2017, but did not elaborate whether there had been unannounced visits.

A former senior U.S. official previously told Reuters that during diplomatic talks many drills had in fact continued but had not been publicized.

South Korea has purchased 40 of its own F-35As from the United States, and is looking to buy another 20. The South Korean air force F-35As will be among the aircraft participating in the joint drills, USFK said.

North Korea has denounced joint exercises as well as South Korea’s weapons purchases as an example of “hostile policies” that prove U.S. offers to negotiate without preconditions are hollow.