South Korea, the United States and Japan on Monday began a combined naval exercise against North Korea's growing submarine threats, Seoul's defense ministry announced.

The three-day training was kicked off in the waters between South Korea and Japan near Jeju Island, involving an Aegis system-equipped guided missile destroyer, several other warships and military choppers, according to the Ministry of National Defense.

It's aimed at securing an "effective response" by the three countries to the North's submarine threats, especially as it's developing submarine-launched ballistic missiles (SLBMs), it added.

South Korea's Navy has dispatched the 4,500-ton Kang Gam Chan destroyer and a Lynx helicopter, with the U.S. sending the USS McCampbell, a destroyer armed with the Aegis ballistic missile defense system, an MH-60 anti-submarine chopper and a P-3 Orion patrol plane. Japan's copter-carrying destroyer, the Sawagiri, also joined the drill.In the practice, they will "search, detect and track a mock submarine, and exchange relevant information," it said.

"The anti-submarine training of the three countries is the first since it was discussed in their Defense Trilateral Talks (DTT) in December," the ministry said, adding it represents their strong determination against North Korea's nuclear and missile threats.

The move comes amid reports that the North is apparently preparing for another nuclear test and more missile launches.

The secretive nation is seen as trying to miniaturize nuclear bombs to fit into its missiles. SLBMs have emerged as a key concern for the South's troops working to establish the Korean Air and Missile Defense (KAMD) system.

The South's arms procurement agency said it's pushing for the purchase of early warning radars in addition to the two Green Pine land-based radars in operation.

"The existing radars are meant mainly to detect ground-launched missiles," a defense official said. "Additional radars are necessary to counter attacks from the enemy's submarines in the West Sea and East Sea."

The Defense Acquisition Program Administration (DAPA) is also considering leasing a military spy satellite from Israel, France or Germany, according to defense sources.

Meanwhile, Seoul and Tokyo signed an agreement last year on sharing military information, called the General Security of Military Information Agreement (GSOMIA).

Japan's media reported earlier that Tokyo and Washington had made an offer for such a joint anti-submarine training shortly after the deal but Seoul turned it down.

The report triggered speculation that South Korea made the reported decision in consideration of relations with China.

South Korean warships stage an anti-submarine drill in this undated file photo provided by the Navy. (Yonhap)

But South Korean officials said it was just a matter of timing, not because of Seoul's rejection.

Seoul and Tokyo have been in a renewed diplomatic stand-off for months over the installation by civic activists of a girl statute in front of Japan's diplomatic mission in Busan to commemorate Korean women forced to serve as sex slaves for Japanese troops during World War II.

In January, Japan recalled its ambassador from Seoul over the incident. He has yet to return. (Yonhap)

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