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.

A passenger watches a TV screen broadcasting a news report on North Korea firing three ballistic missiles into the sea off its east coast, at a railway station in Seoul, South Korea, September 5, 2016. REUTERS/Kim Hong-Ji

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.