SEOUL, Sept. 24 (Yonhap) -- The South Korean military has detected activity in North Korea that suggests it may be preparing to fire a submarine-launched ballistic missile (SLBM) off its eastern coast, officials said Saturday.
The move came one day after the nuclear-powered USS Ronald Reagan aircraft carrier arrived in South Korea's eastern port city of Busan to join the allies' combined military drills later this month.
"We are closely monitoring North Korea's SLBM-related facilities and activities, and maintaining a thorough readiness posture," a military official said.
The military reportedly detected possible signs of preparation for an SLBM test in the North's eastern coastal city of Sinpo on Wednesday and Thursday.
President Yoon Suk-yeol was briefed on the situation earlier Friday by Defense Minister Lee Jong-sup on the flight back from Ottawa, after his weeklong visit to Britain, the United States and Canada, the presidential office said.
Yoon instructed the military to respond immediately in the event Pyongyang makes a provocation, the office said.
Officials in Seoul and Washington have warned North Korea has completed the preparation to resume a nuclear test for the first time since 2017 amid stalled denuclearization talks with the U.S.
A senior U.S. official said Friday it is possible for the North to conduct a nuclear test during U.S. Vice President Kamala Harris' trip to South Korea and Japan next week.
On Wednesday, 38 North, a U.S.-based think tank, said recent satellite imagery of Sinpo showed possible preparations to launch a new submarine capable of firing ballistic missiles.
The satellite imagery, taken Sunday, showed six barges and other vessels gathered around the submarine launch quay at Sinpo South Shipyard, according to 38 North.
"While barges and a dry dock have been occasionally observed around the submarine launch quay at the main construction hall, the presence of six vessels and barges in this area has not been observed before," it said.
The report concluded that the activity suggests preparations to launch a submarine.
Sinpo has production, development and testing facilities for submarines and SLBMs.
North Korea test-fired an SLBM in waters off Sinpo on May 7, three days before Yoon's inauguration. It also launched a mini-SLBM there in October last year.