North Korea launches what it claimed was a hypersonic missile, Jan. 5, in this photo provided by the North Korean government. AP-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.