South Korea's military has detected signs that North Korea may be preparing to test a submarine-launched ballistic missile (SLBM).
Yonhap news agency reported on Saturday, days before a visit by U.S. Vice President Kamala Harris.
The military detected preparations this week in Sinpo, South Hamgyong Province, North Korea, Yonhap reported, citing an unnamed South Korean military source. This is in line with a U.S.-based think tank's report this week, which cited commercial satellite imagery.
South Korean President Yoon Suk Yeol is aware of signs and movements suggesting North Korean provocations, including SLBMs, the presidential office said in a statement on Saturday.
U.S. Vice President Harris is set to visit the region next week and meet with leaders of Japan and South Korea.
A senior U.S. administration official told a briefing on Friday that a nuclear test or other provocation was possible during Harris' trip to the region but that they had no predictions or announcements to make.
A U.S. aircraft carrier arrived in South Korea on Friday for the first time in about four years, joining other military vessels to participate in joint drills with South Korean forces.
North Korea has denounced previous U.S. military deployments and joint drills as rehearsals for war and evidence of hostile policies by Washington and Seoul.