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,” he added.
The defence ministry in Tokyo said the three missiles are 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.
The North’s latest tests sparked strong protests from senior Japanese and US officials.
The launches “are a grave security provocation and can never be permitted”, Japanese Foreign Minister Fumio Kishida told reporters in Tokyo.
“We have lodged a strong protest against North Korea.”
A senior US administration official at the G20 in Hangzhou also condemned the launches as a threat to its allies and to civilian air travel, and vowed diplomatic action against the Pyongyang regime.
“Today’s reckless launches by North Korea pose threats to civil aviation and maritime commerce in the region,” the official said.
Washington would try to “bolster international resolve to hold the DPRK (North Korea) accountable for its provocative actions”.
Monday’s missile launches came hours after South Korean President Park Geun-Hye and Chinese President Xi Jinping met on the sidelines of the summit in Hangzhou.
Ties between South Korea and China have been frosty since Seoul announced plans to deploy a US anti-missile system in July to counter growing nuclear and missile threats from the North.
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.
North Korea has conducted a series of missile tests this year in defiance of UN sanctions imposed after its fourth nuclear test in January. The most recent was a submarine-launched ballistic missile last month.
That missile, fired from off the northeastern port of Sinpo, flew 500 kilometres (around 300 miles) 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. -AFP