North Korea on Wednesday test-fired a ballistic missile towards the Sea of Japan, South Korea's defence ministry said.

The missile was launched from near the western city of Unyul around 07:50 am Wednesday (22:50 GMT Tuesday), it added.

The test follows the launch of three ballistic missiles on July 19 in what the North said were simulated nuclear strikes on the South.

Pyongyang has carried out a series of missile tests this year in defiance of tough UN sanctions, and vowed to take "physical action" against the planned deployment of a US missile defence system in the South announced in July.

UN resolutions prohibit North Korea from developing ballistic missile technology.

Pyongyang has repeatedly warned of pre-emptive nuclear strikes against the South and US targets, although the main focus of its nuclear weapons programme is to develop a credible strike threat against the US mainland.

A series of missile tests this year aimed at backing up that threat led to an agreement between Seoul and Washington to deploy the Terminal High Altitude Area Defence, or THAAD, by the end of the year.

North Korea-US tensions had already been stoked by Pyongyang's fury at Washington's decision to personally target leader Kim Jong-Un with sanctions related to human rights abuses.