North Korea appeared to launch multiple rocket launchers on Sunday, according to Seoul.

The South Korean military identified "flight trajectories" believed to be fired by North Korean artillery, according to Seoul's Joint Chiefs of Staff.

"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.

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"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 convened in response to the North's launching and stated that it was "closely monitoring" the situation in case Pyongyang launched additional missiles.

North Korea has conducted a series of weapons tests this year, including the first full-range intercontinental ballistic missile launch since 2017.

Officials in Washington and Seoul claim that North Korea is planning its seventh nuclear test, which the US has warned will elicit a "swift and forceful" response.

Read next: S. Korea threatens to retaliate against N. Korea if provoked

Last week, in response to concerns that North Korea would perform its first nuclear test in five years, South Korean President Yoon Suk-yeol instructed the military to "promptly and sternly" react in case of any provocation.

According to the presidential office, Yoon, who assumed office in May, chaired his first meeting with senior military commanders and pushed for robust capabilities to thwart North Korea's nuclear and missile programs.

North Korea is reportedly getting ready for its seventh nuclear test and has been testing missiles at an unprecedented rate this year.

The military gathering was held a day after F-35A stealth fighters from the US Air Force made their first publicly announced visit to South Korea since 2017 as the allies and Pyongyang engage in an intensifying cycle of military exercises.

It's worth mentioning that on June 5, the South Korean Joint Chiefs of Staff said the North tested 8 short-range ballistic missiles (SRBMs) from 2 locations near the Sea of Japan. On the following day, South Korea and the US launched 8 surface-to-surface missiles off South Korea's east coast.

The DPRK has recently increased weapons testing to increase its deterrence capabilities. "Only when one is equipped with the formidable striking capabilities, overwhelming military power that cannot be stopped by anyone, one can prevent a war, guarantee the security of the country and contain and put under control all threats and blackmails by the imperialists," Kim said in late March.