South Korea on Friday labelled North Korea's test firing of four short-range missiles a calculated, provocative act timed to coincide with South-US joint military exercises.

North Korea test-fired the missiles into the Sea of Japan on Thursday, three days after the joint drills kicked off in the face of vocal opposition from Pyongyang.

"With the exercises underway, we see the firings as a calculated, provocative act," defence ministry spokesman Kim Min-Seok told journalists.

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

Kim said the tests were of Scud-type missiles at the longer edge of the short-range spectrum, with an estimated reach of 300-800 kilometres (185-500 miles) -- capable of striking any target in the South.

"If the North re-engineers Scuds or tests them, we always undertake a serious analysis to consider counter-measures," he said.

Kim stressed that the annual military drills with the United States would continue as planned.

In Washington, State Department spokeswoman Jen Psaki urged North Korea "to exercise restraint and take steps to improve its relations with its neighbours".

But Pentagon spokesman Colonel Steven Warren acknowledged that such short-range tests did not put the North in breach of international resolutions.

"We view this as an unannounced weapons test that we see somewhat regularly," he told reporters in Washington.

It is not unusual for North Korea to carry out such tests and observers said they were unlikely to trigger a significant rise in military tensions.

Despite the start of the South Korea-US drills on Monday, which the North routinely condemns as rehearsals for invasion, relations between Seoul and Pyongyang are currently enjoying something of a thaw.

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

Pyongyang had initially insisted that the joint exercises be postponed until after the reunions 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.

North Korea has hundreds of short-range missiles and has developed and tested -- with limited success -- several intermediate-range models.

Its claims to have a working inter-continental ballistic missile have been treated with scepticism by most experts, but there is no doubt that it is pushing ahead with an active, ambitious missile development programme.

North Korea's Scud launch violated UN resolutions: US
Washington (AFP) Feb 28, 2014 - North Korea's launch of Scud missiles violated UN resolutions barring the firing of ballistic missiles, the Pentagon said on Friday.

The Defense Department initially said that the missile launch on Thursday was legal, but now says that was incorrect.

"Yesterday, I erroneously noted that these resolutions allow for North Korea to fire short range Scud missiles. That is not the case," Pentagon spokesman Colonel Steven Warren said in a statement.

UN Security Council resolutions 1718 and 1874 prohibit North Korea "from launching any ballistic missile, and this includes any Scud missile," he said.

The resolutions were adopted unanimously by the Security Council in 2006 and 2009 after nuclear tests carried out by Pyongyang.

The firing of the four short-range missiles off the east coast of North Korea coincided with US-South Korea joint military exercises.

Analysts said Pyongyang used the missile launch to convey its anger over the drills, which overlapped with the end of the first reunion in more than three years of families divided by the Korean War.

The Pentagon spokesman said the United States closely such monitors missile tests and called on the North to "refrain from actions that aggravate tensions and instead come into compliance with its international obligations and commitments."