South Korea and the US on Monday began a joint naval drill along the Korean Peninsula’s eastern coast, military officials have said, in a veiled warning to North Korea as it threatens to conduct a nuclear test in defiance of tighter UN Security Council sanctions.

The three-day exercise aims to guard against possible North Korean provocations and involves naval maneuvers, submarine detection, live-fire drills and anti-ballistic missile drills, according to an official at the Joint Chiefs of Staff.

The South Korean Navy deployed 10 vessels, including one 7,600-ton Aegis destroyer, a corvette and its newest Type-214 submarines as well as anti-submarine and maritime surveillance aircrafts and maritime helicopters, for the exercises held near the eastern port city of Pohang, reports Yonhap News Agency.

The US Navy also mobilized the 6,800-ton nuclear submarine USS San Francisco and the 9,800-ton Aegis cruiser for the drills. Both vessels are equipped with long-range cruise missiles.

Pyongyang said the drills showed that the US and South Korea have been plotting to attack North Korea and increased tensions on the volatile peninsula, according to North Korean state media.

It also said Sunday that North Korean leader Kim Jong-Un had made an “important” decision to bolster the army and protect national sovereignty, fueling concerns on an imminent nuclear test.