At least one of 17 ballistic missiles - launched by North Korea just before 9 a.m. Wednesday morning towards the South's coastline - landed less than 40 miles from the city of Sokcho, triggering air-raid sirens and forcing local residents into bomb shelters.

South Korean officials called the launch an "unacceptable" breach of its territory and President Yoon Suk-yeol labelled it an "effective territorial invasion", promising a "swift and firm response".

That response came less than three hours later, when South Korean jets fired three precision air-to-ground missiles off the eastern coast of North Korea.

On Tuesday North Korea's "Supreme Leader" Kim Jong-un threatened the South, vowing it would, "pay the most horrible price in history" if they continued joint military drills with the US. The statement was perceived as a veiled threat to launching a nuclear strike.

The apparent provocation came during a period of national mourning in South Korea, following a crowd stampede on Halloween that led to the death of more than 150 people - mostly children.

On Monday, a US nuclear-powered submarine participated in military drills with the South. The joint exercises have been held since August.

Tensions have escalated between North and South Korea after a series of ballistic missile tests that North Korea has conducted in recent weeks.

Last week, the two countries exchanged warning shots along their disputed western sea boundary and during the previous month, North Korean leader Kim Jong-un oversaw the launch of two long-range strategic cruise missiles.

Today (Thursday), South Korea's military reported that North Korea fired at least one ballistic missile toward its eastern sea, the latest in a series of weapons tests that have raised tensions in the region, The Associated Press reported.

Seoul’s Joint Chiefs of Staff didn’t immediately confirm what type of missile was fired or how far the weapon flew.

Japan’s emergency broadcasting system also confirmed a North Korean missile launch and said the missile flew over and past Japanese territory.

It sent evacuation warnings to residents in the prefectures of Miyagi, Yamagate and Niigata, urging them to stay indoors.

Video: BBC News