North Korea fired three ballistic missiles towards the Sea of Japan on Monday, the South Korean defence ministry said.
A spokesman did not indicate what missile type was tested or how far they flew.
The reported test coincided with the final day of a summit of the 20 leading and emerging economies, including the US and South Korea, in China as world leaders have sought to isolate North Korea over its nuclear provocations.
In Hangzhou, a US official said the United States was "closely monitoring" the situation.
The Pentagon confirmed the launch and said two of the missiles were intermediate range ballistic missiles while the third was still being assessed.
According to South Korean reports, the missile test came shortly after China's president Xi Jinping met with South Korean President Park Geun Hye.
Park said at a meeting with Xi on the sidelines of the summit that North Korea's nuclear tests and missile launches posed a challenge to the South's ties with China, Yonhap news agency reported.
Xi warned Park against stationing US anti-aircraft weapons systems in South Korea. The government in Seoul says these are intended to protect against North Korean missile attacks.
"The wrong approach to the problem would not contribute to strategic stability in the region and could intensify the conflict," Xi said, according to China's Xinhua news agency.
In August, North Korea test-fired a ballistic missile from a submarine.
Tensions have been higher on the Korean peninsula since a series of North Korean tests - including an atomic test - at the start of the year.
The European Union slammed the test as "yet another in a string of serious violations" in a statement released on behalf of the bloc's foreign policy chief, Federica Mogherini.