According to MANA,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 South's military said the missiles were medium-range Rodong-class, launched as a show of force timed to coincide with the G20 summit.
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.
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.