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 the capital Pyongyang just after noon local time (0300 GMT) and flew about 1,000 km (600 miles), hitting Japan's air defense identification zone, the South's Office of the Joint Chiefs of Staff said.
"We are still analyzing details but this is a grave threat to our nation's security, and we express deep concern," the Japan Defence Ministry said in a statement.
The missile launches were the latest in a series of launches by the isolated North this year in violation of U.N. 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 defense.
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 cooperate on monitoring the situation, a Japanese statement said.
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 U.S. President Barack Obama at the Hague to discuss responding to the North's arms program.
The latest missiles were launched from a region called Hwangju and came just hours after the leaders of South Korea and China met on the sidelines of the G20 summit in Hangzhou, China.
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, Yonhap news agency reported earlier.
During the meeting, Xi reaffirmed China's commitment to realizing the denuclearization 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, Yonhap news agency said.
In July, the North launched three missiles in a single day from the western region that flew across the country and into the sea off its east coast, flying about 500 km and 600 km (300-360 miles).