SEOUL, South Korea (AP) -- 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.
The training was arranged to improve the three countries' capacities to respond to underwater security threats posed by North Korea's advancing submarine-launched ballistic missiles and other assets, the statement said. South Korean defense officials said the three countries were to detect and track unmanned South Korean and U.S. underwater vehicles posing as enemy submarines and other assets.
Submarine-launched missiles by North Korea are serious security threats to the United States and its allies because it's harder to spot such launches in advance. In recent year, the North has been testing sophisticated underwater-launched ballistic missiles and pushing to build bigger submarines including a nuclear-powered one.
Last month, North Korea performed a barrage of missile tests in response to the earlier South Korea-U.S. bilateral military drills. The weapons tested included a nuclear-capable underwater drone and a submarine-launched cruise missile, which suggest North Korea is trying to diversify underwater weapons systems.
Photographs in North Korea's state media last week showed leader Kim Jong Un standing near about 10 red-tipped warheads called "Hwasan (volcano)-31" with different serial numbers. A poster on a nearby wall listed eight kinds of short-range weapons that can carry the "Hwasan-31" warhead. The previous test flights of those weapons show they are capable of striking key targets in South Korea, including U.S. military bases there.
Some observers say the warhead's unveiling may be a prelude to a nuclear test as North Korea's last two tests in 2016 and 2017 followed the disclosures of other warheads. If it does conduct a nuclear test, it would be its seventh detonation overall and the first since September 2017.
During the warhead-related event last week, Kim also ordered officials to ramp up bomb fuel production to achieve his stated goal of expanding his country's nuclear arsenal ""exponentially." On Saturday, 38 North, a North Korea-focused website, said that recent commercial satellite imagery of the North's main Yongbyon nuclear complex indicates a high level of activity, such as the continued operation of its 5-megawatt reactor and new construction around the uranium enrichment plant area.
Foreign experts debate whether North Korea has functioning nuclear-armed missiles. But South Korea's defense minister, Lee Jong-Sup, recently told lawmakers that the North's technology to build miniaturized warheads to be mounted on advanced short-range missiles was believed to have made considerable progress.
North Korea could carry out new missile tests to protest the South Korea-U.S.-Japan drills because it views such training as a security threat. North Korean leader Kim Jong Un called the recent South Korea-U.S. exercises "reckless military provocations" that disregarded North Korea's "patience and warning."
In remarks carried in the Defense Ministry statement, Rear Adm. Kim Inho, chief of the South Korean forces involved in the trilateral drills, said "We'll decisively respond to and neutralize any type of provocation by North Korea."
In addition to the anti-submarine drills, the three countries will practice humanitarian search-and-rescue operations, including saving people who fall into the water and treating emergency patients. It would be the three countries' first such training in seven years, the Defense Ministry statement said.