North Korea has fired an unidentified ballistic missile, Seoul's military said on Thursday, in what was Pyongyang's third launch in less than a week, which came just hours after US Vice President Kamala Harris left South Korea.

"North Korea fired an unidentified ballistic missile into East Sea," Seoul's Joint Chiefs of Staff said in a statement, referring to the waters also known as the Sea of Japan.

Japan's coast guard also confirmed a possible ballistic missile launch from North Korea, citing information from Tokyo's defence ministry.

While in South Korea, Harris toured the country's heavily fortified border with the nuclear-armed North, part of a trip aimed at strengthening the security alliance with Seoul.

Washington has about 28,500 troops stationed in South Korea to help protect it from the North, and the allies are conducting a large-scale joint naval exercise this week in a show of force.

Pyongyang conducted two ballistic missile launches in the days before Harris' arrival, continuing a record-breaking streak of weapons tests this year.

The North fired a short-range ballistic missile (SRBM) on Sunday and two SRBMs on Wednesday, Seoul and Tokyo said.

Under Seoul's hawkish new President Yoon Suk-yeol, Seoul and Washington have boosted joint military exercises, which they insist are purely defensive. North Korea sees them as rehearsals for an invasion.

Seoul announced on Thursday that it would hold trilateral anti-submarine drills with Japan and the US, the first such exercises since 2017.

South Korean officials said this weekend they had detected signs Pyongyang could be preparing to fire a submarine-launched ballistic missile. (AFP)