North Korea fired around 10 short-range ballistic missiles toward the sea off its east coast on Thursday, South Korea's military said, in a barrage that has heightened tensions along the heavily armed inter-Korean border.

South Korea's Joint Chiefs of Staff said the missiles were launched at around 12:30 p.m. local time (0330 GMT) from around Wonsan on North Korea's eastern coast and flew about 250 kilometres. Japan's Defence Ministry said North Korea had launched what could be a ballistic missile and that it did not appear to have landed within Japan's Exclusive Economic Zone. Japan's Coast Guard said no damage to vessels had been confirmed.

Seoul and Tokyo respond

South Korea said it was strengthening surveillance and maintaining readiness for additional launches, while closely sharing information with the United States and Japan. South Korea's presidential National Security Office convened an emergency meeting to assess the security implications, and urged North Korea to halt its repeated ballistic missile launches, calling them a serious violation of United Nations Security Council resolutions.

North Korea is barred from ballistic missile tests under Security Council resolutions, though it has continued to develop its missile programme. Thursday's launch was its first reported ballistic missile firing in 17 days and its third launch event this month, according to South Korea's military.

Border tensions and loudspeakers

The launches come as tensions run high ahead of Pyongyang's end-of-month deadline to activate propaganda loudspeakers along the border. Days earlier, the North had threatened to resume front-line propaganda loudspeaker broadcasts and deploy new weapons along the frontier, and the launches followed six days after Seoul proposed military talks aimed at preventing accidental clashes along the border.

Reuters witnesses also observed three North Korean armoured vehicles near the border that South Korean experts identified as self-propelled 122-millimetre howitzers, with their barrels pointed south, adding to the sense of a deliberately raised military posture.

A busy October for missile tests

October has already seen repeated North Korean missile activity. On October 3, the North fired a missile from near its east coast that Japan's Defence Ministry said flew about 680 kilometres at a peak altitude of about 90 kilometres before landing outside Japan's Exclusive Economic Zone. On October 12, Pyongyang fired several ballistic missiles from near its capital, some of which flew more than 900 kilometres, which North Korean state media later described as tests of tactical ballistic missiles, short-range weapons meant for battlefield targets.

The launches also followed leader Kim Jong Un's surprise weekend visit to China, the North's main ally and economic lifeline, to push for greater cooperation, according to reports of the trip. Pyongyang has conducted missile tests on more than 10 occasions this year.

For South Korea, Japan and the United States, the immediate question is whether Thursday's barrage is a one-off show of force tied to the border dispute or the start of a further round of testing. With surveillance stepped up and the loudspeaker deadline approaching, officials in Seoul have warned that further launches remain possible.