North Korea's nuclear test put US President Barack Obama under pressure yesterday to drop his push for direct diplomacy and instead seek tougher international action against the defiant Stalinist state.

The Obama team has suggested recent North Korean provocations amount to a bargaining ploy but former Bush administration hardliner John Bolton and several North Korea analysts offer different, more ominous readings.

Mr Obama himself is now hinting at a tougher stance than simply seeking the restart of six-party nuclear disarmament talks with North Korea.

"The United States and the international community must take action in response," the president said in hastily arranged remarks at the White House ahead of a Memorial Day ceremony for war dead.

Its recent "reckless" actions, including an April 5 ballistic missile test, have defied UN resolutions, the president said. "As a result, North Korea is deepening its own isolation and inviting stronger international pressure."

South Korea said that the North also test fired three short-range missiles on Monday.

US Secretary of State Hillary Clinton stressed the need for a "strong, unified" approach to North Korea as she began consulting her counterparts in the six-party talks about Monday's emergency UN Security Council meeting.

Under a six-party deal with the United States, China, Japan, South Korea and Russia in 2007, North Korea agreed to scrap its weapons-grade nuclear programs for energy aid. The talks lapsed late last year over a dispute over disarmament verification steps.

After the UN Security Council condemned the April missile launch and tightened sanctions, the North vowed to conduct a second nuclear test as well as ballistic missile tests unless the world body apologised.

It also announced that it was quitting the six-way talks and would restart its plutonium-making program.

Mr Bolton, a former US ambassador to the United Nations under President George W. Bush, said the soft Obama line gave North Korea the excuse it needed to carry out another nuclear test after the first one in 2006 fizzled.