South Korea and the U.S. conducted their second joint air drills of the week on Friday, according to Seoul’s defense ministry, a day after North Korea issued a “nuclear for nuclear” warning over the U.S. deployment of strategic assets to the peninsula.

Friday’s air drill took place over the Yellow Sea with South Korean F-35A fighters and U.S. F-22, F-35B and F-16CM fighters. The allies previously conducted an air exercise with similar assets, as well as a U.S. bomber, on Wednesday.