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 defense ministry said.

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

The North fired four more in the afternoon that travelled about 155 kilometers, 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 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. The North test-fired four short-range Scud missiles off its east coast last week and two more missiles on Monday.

South Korea called the missile launches a "reckless provocation" while the US demanded the North show restraint.

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

AFP