North Korea fired a short-range ballistic missile on Sunday, Seoul's military said, the fourth show of force in a week as South Korea and the United States stage major military drills.
Seoul and Washington have ramped up defence cooperation in the face of growing military and nuclear threats from the North, which has conducted a series of increasingly provocative banned weapons tests in recent months.
South Korea and the United States are currently in the middle of 11 days of joint drills known as Freedom Shield, their largest in five years.
North Korea views all such exercises as rehearsals for an invasion and has repeatedly warned it would take "overwhelming" action in response.
"Our military detected one short-range ballistic missile fired from around the Tongchang-ri area in North Pyongan province at 11:05 am (0205 GMT) towards the East Sea," South Korea's Joint Chiefs of Staff (JCS) said, referring to the body of water also known as the Sea of Japan.
The missile flew 800 kilometres and was under analysis by US and South Korean intelligence, the JCS said in a statement, calling the launch "a serious provocation" that violated UN sanctions.
"Our military will maintain a solid readiness posture based on its ability to overwhelmingly respond to any provocation by North Korea, while carrying out intensive and thorough combined exercises and drills," it added.
The US military's Indo-Pacific Command also condemned the launch, saying it highlighted "the destabilising impact" of North Korea's banned weapons programmes.
Tokyo confirmed the launch, with its deputy defence minister Toshiro Ino telling reporters that it had "lodged a vehement protest to and strongly condemned (North Korea) through our embassy in Beijing."
The missile may have flown on an irregular trajectory, and appeared to have fallen outside Japan's exclusive economic zone, Kyodo News reported, citing unnamed government sources.
The latest launch comes a day after North Korean state media reported that more than 800,000 young North Koreans had volunteered to join the army to fight "US imperialists."
The young volunteers were determined to "mercilessly wipe out the war maniacs" and joined the army to "defend the country," the Korean Central News Agency (KCNA) said.
On Thursday, Pyongyang test-fired its largest and most powerful intercontinental ballistic missile, the Hwasong-17 – its second ICBM test this year.
State media described the ICBM launch as a response to the ongoing, "frantic" US-South Korea drills.
Analysts previously said North Korea would likely use the drills as an excuse to carry out more missile launches and perhaps even a nuclear test.
On Saturday, KCNA said the joint drills by the allies were "inching close to the unpardonable red-line."
The ICBM launch followed two short-range ballistic missiles on Tuesday, and two strategic cruise missiles fired from a submarine last Sunday.
The recent flurry of aggression by Pyongyang has pushed Seoul and Tokyo to mend fences over historical disputes and try to boost security cooperation.
Just hours after the ICBM was fired on Thursday, South Korean President Yoon Suk-yeol arrived in Japan for the first full-scale leaders' summit between the countries in 12 years.
Following their summit, Japanese Prime Minister Fumio Kishida said both countries wanted stronger deterrence capacities, and that suspended security and ministerial talks would now resume.
Yoon said the nations would also revive a military intelligence agreement that Seoul paused when relations nosedived.
Last year, North Korea declared itself an "irreversible" nuclear power, and leader Kim Jong-un recently called for an "exponential" increase in weapons production, including tactical nukes.
Kim earlier this month also ordered the North Korean military to intensify drills to prepare for a "real war." (AFP)