North Korea fired two ballistic missiles on Wednesday, one of which exploded immediately after launch, the United States Strategic Command said, according to reports.
The second missile landed in waters off North Korea's east coast, apparently within Japan's Exclusive Economic Zone, Japanese Prime Minister Shinzo Abe said.
Abe called the launch an "outrageous act and a grave threat to our country's national security."
"It was a clear violation of United Nations Security Council resolutions. We lodged a strong protest with North Korea," Abe told reporters.
The missiles were Rodong intermediate-range ballistic missiles and were launched simultaneously near the western North Korean city of Hwangju, South Korea's Yonhap news agency reported.
They were launched around 7:50 am (2320 GMT Tuesday) local time, Yonhap said, citing South Korea's Joint Chiefs of Staff.
In July, South Korea and the United States agreed to deploy an advanced missile defence system in South Korea's Seongju county to counter Pyongyang's developing missile threats, Yonhap said.
The Terminal High Altitude Area Defense (THAAD) system is designed to shoot down a ballistic missile at heights of 40 to 150 kilometres, the report said.
The deployment in South Korea is scheduled for late 2017, it said.
Following the decision to deploy THAAAD, North Korea warned that it would take a "physical counter-action" against the anti-missile shield, Yonhap reported.
North Korea previously fired three ballistic missiles on July 19, including two mid-range Rodong missiles and a shorter-range Scud missile.