It comes as South Korea holds joint military exercises with the US
North Korea has test-fired a submarine-launched ballistic missile, according to the South.
State news agency Yonhap claimed the missile was launched off the North's eastern coast and travelled 300 miles (around 480km) before falling inside Japan's Air Defence Identification Zone.
The missile is said to have been fired shortly before 6am local time Wednesday (about 10pm Irish time on Tuesday).
An unnamed US official said it detected and tracked what it believed to be a KN-11 ballistic missile.
If correct, it is the latest in a series of missile tests by the North - a show of muscle in full defiance of the UN.
Perfecting submarine-launched ballistic missile technology would take its nuclear capabilities to a new level, allowing deployment far beyond the Korean peninsula.
It comes as the state's southern neighbour holds joint military exercises with the US, which the North insists is preparation for an invasion.
North Korea described the drills, in which 75,000 troops are taking part, as an "unpardonable criminal act".
It warned that any violation of territorial sovereignty would result in a "pre-emptive nuclear strike".