The South Korean, U.S. and Japanese navies began their first anti-submarine drills in six months on Monday to boost their coordination against increasing North Korean missile threats, South Korea’s military said. The two-day drills come as North Korea’s recent unveiling of a type of battlefield nuclear warhead prompted worries the country may conduct its first nuclear test since 2017.

The maritime exercises in international waters off South Korea’s southern island of Jeju involved the nuclear-powered USS Nimitz aircraft carrier and naval destroyers from South Korea, the U.S. and Japan, South Korea’s Defense Ministry said in a statement.