The UN Security Council will convene Tuesday to discuss North Korea after the isolated communist state test-fired three ballistic missiles into the sea as world leaders met at the G20 summit in China.

The discussion -- requested by council members Japan and the United States -- is slated to begin at 11:30 am local time (1530 GMT) in New York, where the council will consider a response to the latest missile launches, diplomats said.

North Korea test-fired three ballistic missiles into the sea Monday, South Korea said, in a new show of force as world leaders met in Hangzhou, China.

The missiles were fired into the Sea of Japan (East Sea) from the North's Hwangju county at around 0300 GMT, a spokesman for Seoul's defence ministry said.

The sabre-rattling follows the North's submarine-launched ballistic missile test some two weeks ago.

"They are speculated to be Rodong missiles with a range of 1,000 kilometres (620 miles) and were fired without navigational warning to Japan," the spokesman said in a statement.

"North Korea's ballistic missile launch is a direct violation of UN Security Council resolutions, aimed at showing off its nuclear and missile capabilities during the G20 summit."

The defence ministry in Tokyo said the three missiles were estimated to have fallen into Japan's maritime Exclusive Economic Zone.

"The ministry expresses serious concern over the missile launches as they pose a grave threat to Japan's national security," a ministry statement said.

North Korea has conducted a series of missile tests this year in defiance of UN sanctions imposed after its fourth nuclear test in January.

Last month, it fired a submarine-launched ballistic missile from the northeastern port of Sinpo.

That missile flew 500 kilometres towards Japan, far exceeding the range of the North's previous sub-launched missiles.

The country's leader Kim Jong-Un described the August test as the "greatest success" and said it put the US mainland within striking range.

The launch was widely condemned by the US and other major powers, but analysts saw it as a clear step forward for North Korea's nuclear strike ambitions.

A proven submarine-launched ballistic missile system would allow deployment far beyond the Korean peninsula and a "second-strike" capability in the event of an attack on the North's military bases.

- Strong protests -
The North's latest tests sparked strong protests from Tokyo and Washington.

A senior US administration official at the G20 condemned the launches as a threat to its allies and to civilian air travel, and vowed diplomatic action against the Pyongyang regime.

US State Department spokesman John Kirby strongly denounced the launches as "reckless."

"Today's reckless launches by North Korea threaten civil aviation and maritime commerce in the region," Kirby said in a statement.

Gary Ross, a Pentagon spokesman, also repeated previous comments saying US commitment to the defence of allies South Korea and Japan was "ironclad."

"This provocation only serves to increase the international community's resolve to counter the DPRK's prohibited activities, including through implementing existing UN Security Council sanctions," he said.

"We intend to raise our concerns at the UN to bolster international resolve in holding the DPRK accountable for these actions."

In Tokyo, Japanese Foreign Minister Fumio Kishida told reporters the launches were "a grave security provocation and can never be permitted."

"We have lodged a strong protest against North Korea."

The latest missile launches came hours after South Korean President Park Geun-Hye and Chinese President Xi Jinping met on the sidelines of the G20 summit in Hangzhou.

During the summit, Xi reiterated Beijing's opposition to Seoul's planned deployment of the Terminal High Altitude Area Defence (THAAD) system, arguing that "mishandling" the issue could "intensify disputes" in the region, China's state-run Xinhua news agency said.

Park labelled North Korea's continued provocations as a "challenge" to Seoul-Beijing ties, adding that security threats from Pyongyang were at an "unprecedented level," South Korea's Yonhap news agency said.