(ATTN: ADDS South Korean defense minister visit to U.S. aircraft carrier in penultimate para)
SEOUL, Sept. 24 (Yonhap) -- The South Korean military has detected activities in North Korea that suggest it may be preparing to fire a submarine-launched ballistic missile (SLBM) off its eastern coast, officials said Saturday.
The suspicious signs came as South Korea and the United States are set to hold joint military drills involving a U.S. nuclear-powered aircraft carrier at the end of this month. U.S. Vice President Kamala Harris also plans to visit Seoul on Thursday for talks with President Yoon Suk-yeol.
"We are closely monitoring North Korea's SLBM-related facilities and activities, and maintaining a thorough readiness posture," a military official said.
The military has recently detected possible signs of preparation for an SLBM test in the North's eastern coastal city of Sinpo, according to sources.
Yoon 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.
The president instructed the military to respond immediately in the event Pyongyang makes a provocation, the office said.
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.
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.
Officials in Seoul and Washington have warned Pyongyang has completed the preparation to resume a nuclear test for the first time since 2017 amid stalled denuclearization talks with the U.S.
Harris is scheduled to make a four-day visit to Japan from Monday, leading a presidential delegation to the funeral of former Japanese Prime Minister Shinzo Abe, and then visit Seoul on Thursday to hold a meeting with Yoon.
On Friday, 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.
Defense Minister Lee Jong-sup and U.S. Forces Korea (USFK) Commander Gen. Paul LaCamera jointly visited the aircraft carrier later Saturday, in an apparent indication of the solid alliance between the two nations.
The nuclear-powered USS Annapolis submarine is also expected to take part in the two countries' first major joint exercises involving a U.S. aircraft carrier since 2017.