North Korea test-fired four short-range missiles into the sea Thursday, Seoul's defense ministry said, in an apparent show of force to coincide with South Korea-US joint military exercises.

A ministry spokesman said the missiles, with an estimated range of 200 kilometers, were fired off the east coast of North Korea.

"Our military will maintain tight vigilance in preparation for additional launches or any military provocation from the North," the spokesman said.

North Korea carries out short-range missile tests on a fairly routine basis, and has used them before to display its anger at the annual military exercises.

Observers said the tests were unlikely to trigger a significant rise in military tensions.

"It seems fairly routine," said Kim Yong-hyun, an expert on North Korean affairs at Seoul's Dongguk University.

It's mainly about sending a message - about the drills and also its anger over the recent UN rights report," Kim said.

The South Korea-US drills kicked off Monday, despite vocal opposition from Pyongyang which views them as rehearsals for invasion.

This year, they overlapped with the end of the first reunion for more than three years of families divided by the Korean War (1950-53) - an event that has raised hopes of greater cross-border cooperation.

Pyongyang had initially insisted that the joint exercises be postponed until after the reunion finished on Tuesday. But Seoul refused and - in a rare concession - the North allowed the family gathering on its territory to go ahead as scheduled.

Dongguk University's Kim said North Korea was experienced at calibrating its military provocations, and would probably stop at the short-range missile tests.

"I don't think it will take any further steps and risk escalating tensions. Pyongyang is more interested in seeing some benefit from its compromise on the reunions," he said.

The launches came days after an incursion by a North Korean patrol boast across the disputed Yellow Sea border that has been the scene of brief but bloody naval clashes in the past.

AFP