North Korea fired a volley of rockets from multiple launchers into the sea Tuesday, in an apparent show of force to coincide with South Korea-US joint military exercises, South Korean officials said.

Three rockets were fired in the morning from a multiple launch rocket system, the South's defence ministry said.

The projectiles were launched from the North's eastern port of Wonsan and flew 55 kilometres into the Sea of Japan (East Sea), it said.

The North fired four more in the afternoon that travelled about 155 kilometres, the ministry said, adding it was believed to have been testing two different types of launcher.

"North Korea is displaying a show of force openly against our joint exercises," said ministry spokesman Kim Min-Seok.

South Korea regards the North's mobile multiple rocket launchers as a dangerous because they can strike strategic facilities far south of the border.

Its troops have increased vigilance following a series of North Korean missile test-launches that drew condemnation from South Korea and the United States.

The North test-fired four short-range Scud missiles off its east coast last week, followed by two more missiles on Monday.

South Korea called the missile launches a "reckless provocation" while the United States demanded the North show restraint and abide by UN Security Council resolutions.

The Scuds are at the longer edge of the short-range spectrum, with an estimated reach of 300-800 kilometres -- capable of striking any target in the South.

It is not unusual for North Korea to carry out such tests, which often go unreported by South Korea.

But Washington said the latest missile tests breached UN resolutions that require Pyongyang to abandon its ballistic missile programme.

The missile tests coincide with annual South Korea-US military exercises that started a week ago and run until mid-April.

Pyongyang routinely condemns such joint exercises as rehearsals for an invasion, while Seoul and Washington say they are purely defensive.

Last year the drills coincided with a sharp and unusually protracted surge in military tensions that saw the North issuing apocalyptic threats of pre-emptive nuclear strikes.

By contrast, this year's drills began as cross-border relations were enjoying something of a thaw.

They overlapped with the end of the first reunion for more than three years of families divided by the Korean War -- an event that raised hopes of greater cross-border cooperation.

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

Analysts believe the missile tests reflect Pyongyang's need to flex its muscles in the wake of the compromise over the reunions.

Last week also saw an incursion by a North Korean patrol boat across the disputed Yellow Sea border, the scene of bloody naval clashes in the past.

From Monday South Korea launched a two-day live-fire exercise involving artillery, naval ships and jet fighters to test readiness against any North Korean incursion off the east coast.