North Korea fired several cruise missiles towards the Yellow Sea in the early hours of Saturday, according to the South Korean military.
South Korean and US intelligence authorities were analyzing details of the launch, South Korea's Joint Chiefs of Staff said in a statement.
It was the latest in a series of missile tests and military exercises conducted by the North in recent weeks, including a failed spy satellite launch late last month, prompting fresh criticism from the country's major political parties.
"It's an act of hostility that threatens the peace on the Korean peninsula," a spokesperson for the ruling People Power Party said.
In a statement by state media KCNA, North Korea's foreign ministry accused Japan of raising tensions in the region by developing long-range missiles, but there was no mention of the missile launch.
Japan and the United States are expected to soon agree on jointly developing an interceptor missile to counter hypersonic warheads being developed by China, Russia, and North Korea, Japan's Yomiuri newspaper reported last month.
Seoul announced sanctions on Friday on five North Korean individuals and one company in response to Pyongyang's launch of what it said was a space rocket last month.
On Thursday, state media KCNA reported that Pyongyang had conducted a simulated "scorched-earth" nuclear strike on targets across South Korea, drawing criticism from Seoul.
The joint annual summertime exercises between South Korea and the US, known as the Ulchi Freedom Shield, came to a close on Thursday after an 11-day run featuring air drills with B-1B bombers.
North Korea protested the deployment of the US strategic bombers by firing two ballistic missiles just hours later in retaliation.
Pyongyang has long denounced the drills as a rehearsal for war.