Japan, South Korea and the U.S. have conducted the first joint anti-submarine warfare (ASW) drills involving the three countries amid North Korea’s growing submarine-launched ballistic missile threat.

The three-day drill, which kicked off Monday, was being held in the waters between Japan and South Korea near the island of Jeju and was aimed at developing an “effective response” by the three nations to the North’s submarine activities, especially its improving SLBM capabilities, the South’s Defense Ministry said Monday.

The Maritime Self-Defense Force said in a statement the same day that the drill to “bolster tactical cooperation” involved the helicopter-carrying Sawagiri destroyer. South Korea sent its own destroyer and helicopter while the U.S. dispatched an Aegis-equipped destroyer and a P-3C Orion patrol plane.

Naval vessels will “search, detect and track a mock submarine, and exchange relevant information,” the South Korean Defense Ministry said.

“The anti-submarine training of the three countries is the first since it was discussed in their Defense Trilateral Talks in December,” the Yonhap news agency reported the ministry as saying.

The exercises began the same day Tokyo announced that Japan’s ambassador to South Korea would return to Seoul, ending a months-long row over a statue symbolizing the so-called comfort women, who were forced to provide sex in Japanese military brothels before and during World War II.

The two Asian rivals have faced off over historical issues stemming from Japan’s brutal colonial rule of the Korean Peninsula and wartime history. Tokyo and Seoul are also involved in a bitter territorial dispute over the Takeshima islets, known in South Korea as Dokdo.

The drills also come amid reports that the North is apparently preparing for another nuclear test just weeks after its latest attempt to launch a missile.

The reclusive country is seeking to miniaturize nuclear warheads to fit onto its missiles, and SLBMs have emerged as a potential second-strike capability that enables retaliation against any attack.

In August, a ballistic missile North Korea fired from a submarine flew 500 km over the Sea of Japan into waters inside Japan’s air defense identification zone.

Tokyo has increased its cooperation with Washington and Seoul over the concerns.

In November, Japan and South Korea inked an agreement to share military intelligence on North Korea amid Pyongyang’s ramped-up nuclear and missile programs. The general security of military information agreement (GSOMIA) was widely seen as a breakthrough between the historical rivals and U.S. allies amid the North’s saber rattling.

Together with the U.S., the two countries have beefed-up their defense cooperation, including through ballistic missile detection and via tracking drills.

On March 14, the three dispatched high-tech Aegis missile-defense ships to the same area where the North had lobbed four missiles into waters off Japan’s coast just over a week earlier. Pyongyang called those launches a dress rehearsal for strikes on U.S. military bases in Japan.

Experts say the growing nuclear threat from North Korea has brought Japan and South Korea together, though some have warned not to anticipate closer ties in the near future.

The relationship between South Korea and Japan is such “that it seldom goes forward in a linear, strategically set-out fashion but in reactive response to regional events,” said Corey Wallace, a security analyst at Freie University in Berlin.

“I suspect that is the best we can expect for now,” Wallace said, adding that the U.S. deployment of the Terminal High Altitude Area Defense (THAAD) missile-defense system to South Korea to defend against the North’s missile threat — and China’s opposition to this — played a key role in driving Seoul closer to Tokyo.

“It was important for the South Korea to demonstrate its independence,” Wallace said. “The perceived coercion by China actually played very poorly in Korea this time around, and may have provided a window of softened sentiment regarding cooperation with Japan for the ASW drills.”