North Korea has fired three ballistic missiles into the sea off its east coast, South Korea has said.

The South claimed the show of force was timed to coincide with the G20 summit in neighbouring China.

Seoul said the missiles were fired into the Sea of Japan from the North's Hwangju county.

"They are speculated to be Rodong missiles with a range of 1,000km and were fired without navigational warning to Japan," said a defence ministry spokesman.

"North Korea's ballistic missile launch is a direct violation of UN Security Council resolutions aimed at showing off its nuclear and missile capabilities during the G20 summit," he added.

Sources at Japan's defence ministry told Reuters the missiles are likely to have landed between 190 and 260km west of Hokkaido, Japan's northernmost main island. 

The United States condemned what it described as the "reckless" launches.

It said they were a threat to civil aviation and maritime commerce, adding that it planned to raise concerns about North Korea's nuclear programme at the G20 summit.

China's Foreign Ministry said it hoped the parties involved would avoid any actions that could increase tensions.  

The North regularly engages in missile and rocket tests, especially when the world's eyes are turned to northeast Asia.

World leaders have gathered for the G20 in the city of Hangzhou in eastern China, which is the North's only major ally.

The launch of the missiles came hours after the leaders of South Korea and China met on the sidelines of the summit.

During the meeting South Korean President Park Geun-hye criticised the North for what she called provocations that are hurting ties between Seoul and Beijing.