North Korea has fired a ballistic missile from a submarine but it appears to have failed soon after launch.
The launch comes the day after the US and South Korea pledged to deploy an advanced anti-missile system to counter threats from Pyongyang, and two days after North Korea warned it was planning its toughest response to what it deemed a "declaration of war" by the US.
That followed Washington's blacklisting of the isolated state's leader Kim Jong Un for alleged human rights abuses.
The South's Office of the Joint Chiefs of Staff said in a statement the missile was launched at 11.30am Seoul time (1230 AEST) in waters east of the Korean peninsula.
The missile was likely fired from a submarine as planned but appears to have failed in the early stage of flight, the Joint Chiefs said.
The US Strategic Command said on Saturday it had tracked what it believed was a KN-11 submarine-launched ballistic missile fired from North Korea's east coast port of Sinpo, which then fell into the sea between the North and Japan.
The command said in a statement the missile was tracked over the Sea of Japan, "where initial indications are it fell".
The North American Aerospace Defense Command (NORAD) determined the missile did not pose a threat to North America, it said.
South Korea's Yonhap news agency said the missile's engine successfully ignited but the projectile soon exploded in mid-air at a height of about 10km, and covered not more than a few kilometres across the water.
The South's military declined to confirm those details.
The missile was detected in the sea southeast of the North Korean city of Sinpo, South Korea's military said.
Satellite images indicate Pyongyang is actively trying to develop its submarine-launched ballistic missile program in this area, according to experts.
Neighbouring Japan, the US, and South Korea's military condemned the missile launch as a flagrant violation of UN sanctions.
The missile launch is a "clear challenge to UN Security Council resolutions", Japanese Prime Minister Shinzo Abe said on Saturday, according to Kyodo news agency.
"We should strongly condemn the launch by working with the international community," Abe told reporters.
Abe said the launch did not gravely affect Japan's national security.
The US said it was monitoring and assessing the situation in close co-ordination with its regional allies and partners.
"We strongly condemn North Korea's missile test in violation of UN Security Council Resolutions, which explicitly prohibit North Korea's use of ballistic missile technology," said Gabrielle Price, spokeswoman for the Bureau of East Asian and Pacific Affairs at the US Department of State.
"These actions, and North Korea's continued pursuit of ballistic missile and nuclear weapons capabilities, pose a significant threat to the United States, our allies, and to the stability of the greater Asia-Pacific," she added.
The North has conducted a string of military tests that began in January with its fourth nuclear test and included the launch of a long-range rocket the following month.
The UN Security Council imposed harsh new sanctions on the country in March for its nuclear test and rocket launch.
North Korea rejects the sanctions as infringement of its sovereignty and its right to space exploration.
South Korea and the US said on Friday they would deploy the Terminal High Altitude Area Defense (THAAD) anti-missile system with the US military in South Korea to counter the threat from nuclear-armed North Korea, drawing a sharp and swift protest from neighbouring China, Pyongyang's sole major ally.
Pyongyang also conducted a test of a submarine-launched ballistic missile in April, calling it a "great success" that provided "one more means for powerful nuclear attack".
