A North Korean weapons' test today is believed to be a submarine-launched ballistic missile, South Korea's military said.
The "unidentified ballistic missile" was fired from Sinpo into the sea east of the peninsula, Seoul's Joint Chiefs of Staff said in a statement.
"Our military detected one unidentified short-range ballistic missile presumed to be an SLBM fired by North Korea," according to the statement.
"South Korean and US intelligence are closely analysing for additional detail," it added.
It is understood the United Nations Security Council will hold an emergency closed-door meeting on North Korea tomorrow after a request from the UK and the United States.
Sinpo, where the missile was fired from, is a major naval shipyard and satellite photographs have previously shown submarines at the facility.
North Korea is known to be developing a submarine-launched ballistic missile (SLBM) and previously carried out an underwater launch, although analysts said that one was likely to have been from a submerged platform rather than a submarine.
"There is a high possibility the North launched an SLBM," said Shin Beom-chul, a researcher at the Korea Research Institute for National Strategy.
It comes after the nuclear-armed North, which invaded its neighbour in 1950, in recent weeks tested a long-range cruise missile, a train-launched weapon and what it said was a hypersonic warhead, sparking global concern.
It also mounted a rare weapons exhibition, showcasing the gigantic international ballistic missile (ICBM) revealed at a night-time military parade last year.
"The fundamental reason for the North's provocation is because the US is not changing its position on talks," said Mr Shin.
"Pyongyang is trying to demonstrate that it can carry out a bigger provocation."
Opening the weapons exhibition, leader Kim Jong-Un blamed the United States for tensions, dismissing Washington's assertions that it does not have hostile intentions. Kim has overseen rapid progress in the North's military technology, at the cost of international sanctions.
Something of a regional arms race is developing on the peninsula, with the South last month testing its first SLBM, putting it among the elite group of nations that have demonstrated proven technology, and unveiling a supersonic cruise missile.
Following today's launch, the South's presidential office said it was convening a meeting of the National Security Council.
Japanese Prime Minister Fumio Kishida said two ballistic missiles had been fired, called the launch "very regrettable".