North Korea test-fired two ballistic missiles Wednesday, as US President Barack Obama hosted a landmark Japan-South Korea summit and pledged "unwavering commitment" to Tokyo and Seoul's security concerns in the face of Pyongyang's nuclear threat.
A military vehicle carries a missile during a parade in Pyongyang on April 15, 2012
South Korea's defence ministry said both missiles flew 650 kilometres (400 miles) into the Sea of Japan, upping the ante after a series of short-range missile and rocket launches by the North in recent weeks.
A ministry spokesman told AFP the missiles were believed to be Rodong variants, which are considered medium-range at their maximum reach of between 1,000 and 1,500 kilometres.
UN Security Council resolutions 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.
The Scuds are at the longer edge of the short-range spectrum, with an estimated reach of 300-800 kilometres -- capable of striking any target in the South.
South Korea condemned the Scud launches as a "reckless provocation" but both Seoul and Washington stopped short of calling for UN sanctions, given the short-range of the missiles and a recent easing of North-South tensions.
The North Korean military had defended the tests as "ordinary military practice".
If the missiles launched on Wednesday are confirmed to be Rodongs, observers said it would lay down a challenge to the international community to consider sanctions.
The last time North Korea was believed to have tested a Rodong missile, also known as Nodong, was in July 2009, following UN condemnation of its second nuclear test in May of the same year.
- 'Unified response' -
The latest launches were clearly timed to coincide with the summit in The Hague.
The talks between Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-Hye came as Obama sought to help repair strained ties between two of the United States' closest Asian allies and key partners in the effort to curb North Korea's nuclear programme.
"Over the last five years, close cooperation between the three countries succeeded in changing the game with North Korea," Obama said.
"Our trilateral cooperation has sent a strong signal to Pyongyang that its provocations and threats will be met with a unified response," he added.
Relations between Tokyo and Seoul are at their lowest ebb in years, mired in emotive issues linked to Japan's 1910-45 colonial rule and a territorial dispute, as well as Japan's use of South Korean "comfort women" as sex slaves in wartime brothels.
Washington has placed a priority on improving those ties, fearful that an open rift would undermine the three countries' united stance against North Korea's nuclear ambitions.
North Korea has hundreds of short-range missiles and has developed and tested -- with limited success -- several intermediate-range models.
Its claims to have a working inter-continental ballistic missile have been treated with scepticism by most experts, but there is no doubt that it is pushing ahead with an active, ambitious missile development programme.
South Korea's military exercises with the United States, which will wind up next month, are an annual source of North-South tension, with Pyongyang denouncing them as rehearsals for invasion.
Last year they coincided with an unusually sharp and protracted surge in military tensions that saw North Korea issuing apocalyptic threats of pre-emptive nuclear strikes.
By contrast, this year's drills began as relations between Seoul and Pyongyang were enjoying something of a thaw.
They overlapped with the end of the first reunion for more than three years of families divided by the Korean War -- an event that raised hopes of greater cross-border cooperation.
Pyongyang had initially insisted that the joint exercises be postponed until after the reunions finished. But Seoul refused and -- in a rare concession -- the North allowed the family gatherings on its territory to go ahead as scheduled.
Most analysts believe the subsequent missile and rocket tests reflect Pyongyang's need to flex its muscles in the wake of the reunion compromise.