Pyongyang's latest provocation comes as concern grows over the whereabouts and wellbeing of a US soldier who made an unauthorised dash into North Korean territory earlier this week, with the reclusive regime so far remaining tight-lipped about the incident.

Saturday's launches are just the latest in a series by Pyongyang, and come as Seoul and Washington ramp up defence cooperation, with relations between the two Koreas nearing an all-time low.

The cruise missile launches took place around 4 am (1900 GMT) on Saturday, Seoul's military said.

"South Korean and US intelligence authorities were analysing the launches while monitoring signs of additional activities," the JCS said.

North Korea had fired two ballistic missiles into the sea off its opposite coast toward Japan just three days earlier.

Diplomacy between Pyongyang and Seoul has stalled, and North Korean leader Kim Jong Un has called for ramping up weapons development, including tactical nukes.

In response, Seoul and Washington have staged joint military exercises with advanced stealth jets and US strategic assets, while an American nuclear-armed submarine this week made a South Korean port call for the first time in decades.

- 'End of the regime' -

On his visit to the submarine on Wednesday, South Korean President Yoon Suk Yeol warned Pyongyang it would face the "end of the regime" if it attacked the South with nuclear weapons.

"I feel assured having seen the US' most important nuclear strategic asset here," he said.

On Thursday, North Korean defence minister Kang Sun Nam said the Ohio-class submarine's deployment might have fallen "under the conditions of the use of nuclear weapons specified in the DPRK law on the nuclear force policy", using an acronym for the North's official name.

North Korea last year adopted a sweeping nuclear law setting out an array of scenarios in which it could use its nukes, including pre-emptive strikes if threatened.

A day later, South Korea's defence ministry reiterated that any use of nuclear weapons by the North would prompt an "immediate and decisive response" resulting in the Kim regime's demise.

Saturday's launches came just days after US soldier Travis King was believed to have been taken into North Korean custody after he broke away from a tour group visiting the Demilitarized Zone.

The United States has said it is "very concerned" about Private Second Class King's treatment, and as of Saturday, Pyongyang had yet to respond to inquiries about the soldier.

King had been due to return to the United States to face military discipline after serving jail time in South Korea for assault. Instead he left the airport before he was set to depart, joined a tour of the border and fled across it.

- Sanctions 'concerns' -

Meanwhile, the Group of Seven nations, the European Union and three other countries have urged China to expel oil tankers from its waters that appear to be taking fuel to North Korea in defiance of UN sanctions, according to a letter seen by AFP on Friday.

"We have concerns regarding the continuing presence of multiple oil tankers... that use your territorial waters in Sansha Bay as refuge to facilitate their trade of sanctioned petroleum products to the DPRK," read the letter, which was signed by the countries' ambassadors and addressed to China's UN envoy Zhang Jun.

In the letter, the ambassadors told Zhang they "would like to provide your government with additional information and satellite imagery" supporting the allegations, while calling on China to deny the vessels "all services, and ultimately expel them from your waters".

A source said the letter had been sent to the Chinese envoy, though there was no confirmation it had been received.

North Korea has been subject to UN sanctions since 2006 over its missile launches and nuclear programs, with additional sanctions in 2017 limiting its crude oil imports.