North Korea fired the unidentified projectile towards the sea to its east on Thursday afternoon, South Korea’s Joint Chiefs of Staff (JSC) said.
They added that the South Korean military is maintaining readiness while monitoring the situation.
It comes after North Korea said on October 3 that it had successfully test-fired a new submarine-launched ballistic missile from the sea to contain external threats and bolster self-defence.
Earlier this week, the rogue nation said it is running out of patience with the US over denuclearisation.
In a statement published by Pyongyang’s official Korean Central News Agency, senior North Korean official Kim Yong Chol said there has been no substantial progress despite the friendly relationship between leader Kim Jong Un and US President Donald Trump.
He said the persisting hostility means “there can be the exchange of fire at any moment”.
A summit in Vietnam in February between the two leaders broke down without an agreement.
This is a breaking story. More to follow.