North Korea fired what appeared to be a medium-range ballistic missile into the sea Friday, just days after leader Kim Jong-Un ordered further nuclear warhead and missile tests, South Korea's defence ministry said.

A ministry spokesman said the missile was launched from Sukchon in the country's southwest at 5:55 am (2055 GMT Thursday) and flew 800 kilometres (500 miles) into the East Sea, also called the Sea of Japan.

He did not confirm the type of missile, but South Korea's Yonhap news agency cited military sources as saying it was a Rodong missile, a scaled-up Scud variant with a maximum range of around 1,300 kilometres.

Military tensions have been soaring on the divided Korean peninsula since the North carried out its fourth nuclear test on January 6, followed a month later by a long-range rocket launch that was widely seen as a disguised ballistic missile test.

The UN Security Council responded earlier this month by imposing its toughest sanctions on North Korea to date.

US President Barack Obama signed an order on Wednesday implementing the UN sanctions, as well as a series of unilateral US sanctions adopted by Congress.

- Daily threats -

Pyongyang, meanwhile, has maintained a daily barrage of nuclear strike threats against both Seoul and Washington, ostensibly over ongoing, large-scale South Korea-US military drills that the North sees as provocative rehearsals for invasion.

To register its anger at the joint exercises, the North fired two short-range missiles into the East Sea on March 10.

A few days later, North Korean President Kim Jong-Un announced that a nuclear warhead explosion test and firings of "several kinds" of ballistic rockets would be carried out "in a short time".

Existing UN sanctions ban North Korea from the use of any ballistic missile test, although short-range launches tend to go unpunished.

A Rodong test would be more provocative, given its greater range which makes it capable of hitting most of Japan.

The last Rodong test was in March 2014, when two of the missiles were fired into the East Sea.

While North Korea is known to have a small stockpile of nuclear weapons, its ability to deliver them accurately to a chosen target on the tip of a ballistic missile has been a subject of heated debate.

There are numerous question marks over the North's weapons delivery systems, with many experts believing it is still years from developing a working inter-continental ballistic missile (ICBM) that could strike the continental United States.

- Re-entry test -

Kim's announcement of further tests on Tuesday cams as he monitored what was described as the successful simulated test of the warhead re-entry technology required for such a long-range nuclear attack.

The test was a complete success, state media said, and provided a "sure guarantee" of the warhead's ability to withstand the intense heat and vibration of re-entry -- a major step in the North's push towards a genuine ICBM nuclear strike capability.

South Korea said it doubted the North had mastered re-entry technology, although it was less sceptical a few days before when Kim said it had miniaturised a nuclear warhead that could fit on a missile.

Earlier this week, South Korean President Park Geun-Hye said North Korea's endless threats and provocative behaviour reflected a "sense of crisis" in Pyongyang at its increasing diplomatic and economic isolation.

"If North Korea continues its provocations and confrontation with the international community and does not walk the path of change, it will walk the path of self-destruction," Park said.

N. Korea fires ballistic missile into sea: Seoul