WASHINGTON (AFP) - The United States has said that North Korea's test firing of two medium-range missiles into the Sea of Japan represents "a troubling and provocative escalation" on the Korean Peninsula.

America is closely coordinating with allies and partners, including in the UN Security Council, "to take the appropriate measures in response" to Pyongyang's latest "violation of UN Security Council resolutions", the State Department said.

"We urge North Korea to exercise restraint and refrain from further threatening actions," said deputy spokeswoman Marie Harf in a statement following the missile launch which the US is taking "very seriously".

The North's action came as US President Barack Obama hosted a landmark Japan-South Korea summit and pledged his "unwavering commitment" to Tokyo and Seoul in the face of Pyongyang's nuclear threat.

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 that South Korea is conducting with the United States.

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

The North Korean military had defended the tests as "ordinary military practice".