US President Barack Obama pledged "unwavering commitment" to Tokyo and Seoul's security concerns in the face of nuclear-armed North Korea on Tuesday as Pyongyang reportedly test-fired two ballistic missiles.

"The US commitment to the security of both Japan and the Republic of Korea is unwavering... and a nuclear North Korea is unacceptable," Obama said at a landmark Japan-South Korea summit he hosted in The Hague.

A scant few hours later, South Korea's Yonhap news agency reported that North Korea test-fired two medium-range missiles into the sea early Wednesday.

Pyongyang has carried out a series of short-range missile tests in recent days in apparent protest at joint drills between Seoul and Washington.

North Korea threatened to take nuclear "measures" if Washington does not end what the communist state calls "provocations".

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 partners.

"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.

"Japan and South Korea have common challenges, and we will continue to have communication at various levels," Abe told AFP after the talks.

"Today was the first step to building the future oriented relations between Japan and South Korea," he said.

The three-way meeting at the US ambassador's residence in The Hague -- designed to discuss North Korean threats -- is considered a diplomatic breakthrough.

South Korean leader Park reiterated her view that the nuclear issue posed a major threat to peace and stability and that it was vital for the international community to have a united response.

Although not a one-on-one encounter, the talks are a significant step forward as Park had repeatedly ruled out a summit with Abe until Tokyo demonstrates sincere repentance for "past wrongdoings".

Recent surveys in South Korea have shown that the Japanese leader is even more unpopular with South Koreans than North Korean supremo Kim Jong-Un.

But prospects for a meeting between Park and Abe rose earlier this month after the Japanese leader promised to honour Tokyo's two previous apologies over its colonial past, issued in 1993 and 1995.

- 'How serious are they?' -

Japanese politicians express exasperation at the repeated requests for contrition, pointing to numerous apologies as well as a 1965 agreement that normalised relations and included a large payment to Seoul.

The situation was exacerbated by Abe's visit to a controversial war shrine in December that drew strong protests from South Korea and China, which also suffered during Japan's past colonial aggression.

"The Japanese government must offer clear signals and put measures in place to restore mutual trust," Park told German daily Frankfurter Allgemeine Zeitung.

Park said recent comments by the Japanese government that it would uphold the apology statement were "reassuring" but added: "The real question is: 'how serious are they?'"

The rift has been viewed with growing alarm in Washington. South Korea and Japan are the two major US military allies in Asia, and key to Washington's strategic "pivot" to the region.

"It was very meaningful that the three countries agreed to collaborate on security in east Asia, centering on the North Korea issue, given that these three countries share values and security interests, because both countries are allies of the US," Japanese foreign ministry spokesman Masaru Sato told AFP.

In a conversation after the three-way summit, Obama and Abe agreed to accelerate negotiations for both countries to join the Trans-Pacific Partnership trade agreement, Sato said.