North Korea fired three ballistic missiles into the sea off its east coast on Monday, South Korea's military said, as the leaders of the Group of 20 major economies held a summit in China, the North's main diplomatic ally.
The missiles were fired from a region south of capital Pyongyang just after noon local time and flew about 1000km, hitting Japan's air defence identification zone.
"We are still analysing details but this is a grave threat to our nation's security, and we express deep concern," the Japan defence ministry said.
The missile launches were the latest in a series of launches by North Korea this year in violation of UN Security Council resolutions, supported by China, that ban all ballistic missile-related activities by the North.
Pyongyang rejects the ban as infringing its sovereign right to pursue a space program and self defence.
Shortly after the missile launches, South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe met on the sidelines of the G20 summit and agreed to co-operate on monitoring the situation.
The South's military said the missiles were medium-range Rodong-class, launched as a show of force timed to coincide with the G20 summit.
In 2014, the North fired two Rodong medium-range missiles just as Park and Abe were meeting US President Barack Obama to discuss responding to the North's arms program.
South Korea's Park told Chinese President Xi Jinping that the North's fourth nuclear test and its ballistic missile launches this year threatened regional peace and posed a challenge to South Korea's ties with China.
During the meeting, Xi reaffirmed China's commitment to realising the denuclearisation of the Korean peninsula, China's state news agency Xinhua reported on Monday.
Xi also told Park that Beijing opposed the proposed deployment of a THAAD anti-missile system in South Korea, which Seoul and the United States have said is designed to counter an increasing missile threat from North Korea.
Park said that a THAAD deployment would not threaten any other country's security interests and would not be needed if the North's nuclear issue was resolved.