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

A ministry spokesman said the missiles, with an estimated range of 200km (125 miles), 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 regularly carries out short-range missile tests, 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.

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

This year they overlapped with the reunion of families divided by the Korean war – an event that has raised hopes of greater cross-border co-operation after a three-year hiatus.

Pyongyang initially insisted 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 gatherings on its territory to go ahead as scheduled.