"The South Korean military has detected multiple cruise missiles launched into the East Sea" by North Korea, Seoul's Joint Chiefs of Staff said in a statement, referring to the body of water also known as the Sea of Japan.

"Detailed specifications are being analysed by South Korea-US intelligence authorities," it added.

After a record-breaking year of weapons tests and growing nuclear threats from Pyongyang, Seoul and Washington have ramped up security cooperation, and on March 13 kicked off their largest joint military drills in five years.

Known as Freedom Shield, the drills run for at least 10 days.

South Korea's military said it would "successfully complete the planned joint Freedom Shield drills under a firm joint defence posture."

North Korea views all such exercises as rehearsals for invasion, saying they could be seen as a "declaration of war" and has repeatedly threatened to take "overwhelming" action in response.

Wednesday's launch comes about a week after Pyongyang test-fired its largest and most powerful missile, a Hwasong-17 -- its second ICBM test this year.

The North's state media described the ICBM launch as a response to the ongoing, "frantic" US-South Korea drills.

North Korea declared itself last year an "irreversible" nuclear power and leader Kim Jong Un recently called for an "exponential" increase in weapons production, including tactical nuclear weapons.

Kim also ordered the North Korean military this month to intensify drills to prepare for a "real war".