North Korea fired what appeared to be multiple rocket launchers on Sunday, Seoul said, according to the AFP news agency.
The South Korean military detected "flight trajectories" that were suspected to be shots from North Korean artillery, Seoul's Joint Chiefs of Staff said, according to the report.
"Our military spotted flight trajectories that are suspected to be North Korea's multiple rocket launchers from around 18:21 to 18:37 pm today," said the Joint Chiefs of Staff in a text message to reporters.
"Our military has strengthened surveillance and vigilance, and maintained a thorough readiness posture while keeping close US-South Korea cooperation," it added, without further details.
Seoul authorities did not share how many trajectories were detected in total.
The presidential National Security Office held a meeting over the North's firing, and said it was "closely monitoring" the situation in case of additional launches by Pyongyang.
North Korea has conducted more than a dozen weapons tests since January, including the firing an intercontinental ballistic missile at full range for the first time since 2017.
In May, it fired a volley of missiles, including possibly its largest intercontinental ballistic missile.
North Korea has ramped up its missile tests ever since talks with Washington on denuclearization failed.
Former US President Donald Trump tried to reach an agreement with North Korea while in office. Kim and Trump met in Hanoi in 2019 for a summit that left nuclear talks at a standstill.
The pair had met three times since June 2018 but made little progress towards denuclearization.
The Biden administration reached out to North Korea shortly after taking office, but the country did not respond to those overtures.
Last month, the United States and South Korea warned of a “swift response” if North Korea conducts a nuclear test, but also offered talks as a way out.