North Korea fired a ballistic missile from a submarine this morning but it appears to have failed soon after launch, according to South Korea's military.
The launch comes at the end of a week of sharply rising tensions on the peninsula.
It is only a day after the US and South Korea pledged to deploy an advanced anti-missile system to
counter threats from North Korea, and two days after North Korea warned it was planning its toughest response to what it deemed a "declaration of war" by the United States.
That followed The US'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 that the missile was launched at about 11.30am local time (3.30am Irish time) 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.
Neighbouring Japan, the United States, and South Korea's military condemned the missile launch as a flagrant violation of UN sanctions.
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 citing its policy of not publicly commenting on intelligence matters.
The missile was detected in the sea southeast of the North Korean city of Sinpo, South Korea's military said. Satellite images indicate North Korea is actively trying to develop its submarine-launched ballistic missile programme in this area, according to experts.
The US Strategic Command, whose mission is to detect and prevent strategic attacks against the United States and its allies, said it had detected what it believed was a KN-11 submarine-launched ballistic missile.
It was fired from North Korea's east coast port of Sinpo and then fell into the sea between there and Japan, the command said in a statement.
Japanese Prime Minister Shinzo Abe said the missile launch is a "clear challenge to UN Security Council resolutions".
"We should strongly condemn the launch by working with the international community," Mr Abe told reporters.
However, he said the launch did not gravely affect Japan's national security.
The US said it was monitoring and assessing the situation in close coordination with its regional allies and partners.