N Korea test-fires 30 missiles into sea

North Korea has test-fired 30 short-range missiles into the sea, South Korea's Joint Chiefs of Staff say, in the latest in a series of launches that have provoked criticism from Seoul and Washington.

"North Korea fired off 30 short-range missiles between 4am and 6am [6am-8am AEDT] this morning from its east coast into the Sea of Japan [East Sea]," a spokesman for the Joint Chiefs said.

"The missiles are estimated to have flown about 60 kilometres."

South Korea urged North Korea earlier this week to stop what it called "provocative" and potentially dangerous tests, a day after Pyongyang test-fired 25 projectiles into the sea.

The show of force is apparently intended to express anger at the South's continuing joint military exercises with its ally the US. It is not unusual for Pyongyang to carry out such tests but there has been a spate of them in recent weeks.

"The North should stop actions that cause military tension and unnerve its neighbours," Seoul's defence ministry spokesman Kim Min-Seok told reporters on Monday.

"Provocative action made without any prior notifications ... can pose significant danger to sea vessels and aircraft passing by the area," he added.

Beijing expressed concern earlier this month after the North test-fired a rocket into the flight path of a Chinese airliner.

The annual South Korean-US military drills started in late February and will run until mid-April.

The North has habitually criticised the Key Resolve and Foal Eagle exercises, along with other military drills south of the border, as rehearsals for an invasion.

Seoul and Washington say they are purely defensive.

Earlier this month, the North's powerful National Defence Commission threatened to demonstrate its nuclear deterrent in the face of what it called US hostility.

But Seoul's defence ministry said there was no sign of an imminent nuclear test by the North, which staged three atomic tests in 2006, 2009 and last year.