The United States said that North Korea's test firing of two medium-range missiles into the Sea of Japan yesterday represented "a troubling and provocative escalation" on the Korean peninsula.
America is closely co-ordinating 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, adding the US was taking the launch "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.
South Korea's defence ministry said both missiles flew 650 kilometres into the Sea of Japan, upping the ante after a series of shorter-range launches in recent weeks. Ministry spokesman Kim Min-seok identified them as Rodong-class missiles with a maximum range of 1,000-1,500 kilometres.
"This missile is capable of hitting not only most of Japan but also Russia and China," Kim said.
North Korea last tested a Rodong missile in July 2009, following UN condemnation of its second nuclear test in May.
Kim said the tests were clearly timed to coincide with Tuesday's summit in The Hague between Obama, Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye.
"The North is boasting its military capability to grab the attention of the international community," Kim said.