North Korea on Wednesday test-fired two Nodong-type ballistic missiles into the sea, upping the ante after a series of short-range missile and rocket launches in recent weeks, the South's defence ministry said.

The tests came as US President Barack Obama hosted a landmark Japan-South Korea summit in the Hague and pledged "unwavering commitment" to Tokyo and Seoul's security concerns in the face of nuclear-armed North Korea.

The missiles were both fired in the early hours of Wednesday and flew 650 kilometres (400 miles) into the Sea of Japan (East Sea), a defence ministry spokesman told AFP.

He said the missiles were believed to be Nodong variants, which are considered medium-range at their maximum reach of between 1,000 and 1,500 kilometres.

UN Security Council resolution prohibit North Korea from conducting any ballistic missile tests.

Over the past four weeks, North Korea has conducted multiple launches of short-range Scud missiles and rockets to coincide with the annual joint military drills South Korea is conducting with the United States.

South Korea condemned the Scud launches as a "reckless provocation" but both Seoul and Washington had stopped short of calling for UN sanctions, given the short-range of the missiles and a recent easing of North-South tensions.

If the missiles launched on Wednesday are confirmed to be Nodongs, observers said the reaction could be stronger.

The last time North Korea was believed to have tested a Nodong missile was in July 2009.