South Korea has detected signs that North Korea is preparing to launch a ballistic missile from a submarine, the South Korean presidential office said Saturday, a day after a U.S. aircraft carrier arrived for joint drills.

The South Korean military said there is activity in the North suggesting preparations for the firing of a short-range SLBM off its eastern coastal city of Sinpo, according to South Korea's Yonhap News Agency.

Sinpo has production, development and testing facilities for SLBMs, according to Yonhap.

The North has repeatedly conducted ballistic missile tests since the start of this year, and there are concerns that North Korea may be about to conduct what would be its seventh nuclear test and first since September 2017.

South Korean Defense Minister Lee Jong Sup briefed President Yoon Suk Yeol on the situation on Friday on Yoon's flight back from Canada, with Yoon ordering the military to respond immediately to any provocations by North Korea, according to Yonhap.

It is rare for the presidential office to announce signs of North Korea preparing for a possible missile launch, with such information usually released by the country's military.

The Ronald Reagan arrived in the South Korean port city of Busan on Friday for the first time in nearly five years to join naval exercises by the two allies.

A senior U.S. official told Yonhap that there is a possibility the North may conduct a nuclear test when U.S. Vice President Kamala Harris visits South Korea and Japan next week.

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