The South Korean military said on Monday that North Korea launched a missile from a submarine near Sinpo, South Hamgyong Province the previous day.

The Joint Chiefs of Staff(JCS) said it detected an unidentified missile test-fired by a North Korean submarine in the waters near the eastern coastal city on Sunday morning.

The intelligence authorities of South Korea and the U.S. are working on analyzing the details of the launch, including the range, speed and altitude of the missile.

The South’s military also raised its monitoring and vigilance levels and is maintaining full defense readiness while cooperating closely with the U.S., the JCS said.

The North has developed submarine-launched ballistic missiles and based its submarines in Sinpo.

The launch came three days after the regime fired six short-range ballistic missiles from the western port city of Nampo toward the Yellow Sea, preceding a massive joint military exercise by Seoul and Washington that began on Monday.