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The South Korean and U.S. navies began a three-day exercise Monday in the Sea of Japan amid signs North Korea plans a nuclear test, military officials said.

The drill to test combat readiness of the two allies was also seen as a warning to North Korea, Yonhap News Agency reported.

U.S. war ships participating in the joint exercise off the eastern port city of Phoang included the 6,800-ton nuclear submarine USS San Francisco and the 9,800-ton Aegis cruiser Shilo. Ten South Korean naval vessels were part of the war games, including a 7,600-ton Aegis destroyer, a corvette and its newest Type-214 submarines as well as anti-submarine and maritime surveillance aircraft and maritime helicopters, Yonhap said.

"The exercise involves naval maneuvers, submarine detection, live-fire drills and anti-ballistic missile drills," an official at the South Korea Joint Chiefs of Staff told Yonhap.

Last week, Gen. Jung Seung-jo, South Korean JCS chairman, said the drill aimed to guard against possible North Korean provocations involving submarines.

The North has threatened to conduct its third nuclear test after the U.N. Security Council last month approved a resolution to tighten sanctions against the Communist country for its December long-range rocket launch. Its first two tests were in 2006 and 2009 that resulted in U.N. sanctions.

Yonhap said South Korean and U.S. intelligence officials have been analyzing daily updates from satellite imagery of the North's Punggye-ri nuclear test site, where the third underground test could be held.