"Our military is monitoring the situation in case of additional launches and maintaining a readiness posture," the JCS noted.

The presidential office Cheong Wa Dae held an emergency meeting to discuss the latest launch, according to its spokesperson Ko Min-jung.

"We see the launches as a violation of the spirit of the inter-Korean military agreement," defense ministry spokesperson Choi Hyun-soo told a regular briefing, referring to the Comprehensive Military Agreement signed in September last year. 

In Washington, a senior US official said it is continuing to monitor the situation and is consulting closely with its allies regarding the matter.

It is the fourth such launch in less than two weeks.

The first recent launch took place on July 25 when the North fired two rounds of a new type of ballistic missile for the first time in 77 days. The communist nation conducted additional launches on July 31 and Aug. 2.

South Korean and US officials said that all three previous launches involved a new type of short-range ballistic missile, known as its version of Russia's Iskander codenamed KN-23. But the North said the last two tests, conducted on July 31 and Aug. 2, involved a new "multiple launch guided rocket system."

What makes Tuesday's launches stand out is the fact that they were conducted from a western region and the projectiles flew all the way across the peninsula into the East Sea, while the three previous launches were carried out from the east coast.

"Now that North Korea has some level of confidence in the stability of this new weapon after the recent launches, it appears to have brought it to the western region for additional testing by tuning its flight range and altitude," Kim Dong-yup, a professor at Kyungnam University's Far East Institute, said.

On May 9, North Korea fired two apparent short-range missiles from its northwestern area of Kusong in an easterly direction, five days after it launched a barrage of projectiles, involving at least one short-range missile, from its eastern coastal town of Wonsan.

UN Security Council resolutions ban North Korea from launching ballistic missiles of any kind.

Tuesday's firing came a day after South Korea and the United States began their combined military exercise as scheduled despite North Korea's repeated warnings against it.

The allies' "crisis management staff training" that kicked off Monday is a preliminary session in the runup to their summertime command post exercise, according to military sources.

The exercise, which is expected to continue for about three weeks, is meant to test South Korea's initial operational capability (IOC) for the envisioned transfer of the wartime operational control from Washington to Seoul, they added.

The allies have kept the joint exercise low key, with neither side officially declaring its beginning or disclosing details, including its name and duration. But Defense Minister Jeong Kyeong-doo confirmed during a parliamentary session on Monday that the drill is "underway."

In a fresh warning after the firing Tuesday, North Korea's foreign ministry said in a statement that it could seek "a new road," other than engagement, calling the joint exercise a violation of a series of joint agreements they signed with the North.

"The US and South Korean authorities remain outwardly talkative about dialogue. But when they sit back, they sharpen a sword to do us harm," the ministry said, according to the North's Korean Central News Agency in an English dispatch.

"If the US and South Korean authorities trust to luck, disregarding our repeated warnings, we will make them pay heavy price which will in turn make them very much difficult," it added.

Nuclear negotiations have been at a standstill since the no-deal summit between US President Donald Trump and North Korean leader Kim Jong-un in Hanoi in Feb. After an impromptu meeting between the two leaders at the inter-Korean border village of Panmunjom in June, however, the two sides agreed to resume working-level talks, though the talks have not taken place so far. (Yonhap)