The U.S. and South Korea staged their first combined military training involving an American aircraft carrier in more than four years, Seoul’s military announced on Saturday, amid reports that North Korea will soon conduct another nuclear test.

U.S. and ROK navies began the three-day exercise on Thursday in international waters off the Japanese island of Okinawa, according to Seoul’s Joint Chiefs of Staff (JCS) in a statement. The exercises included air defense, anti-ship, anti-submarine and maritime interdiction operations.