President Barack Obama didn't disclose what further steps the countries might take as he met with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye on the sidelines of a nuclear security summit.

But he said the countries had directed their teams to work together to help bring about a denuclearized Korean Peninsula.

Park, whose country has been repeatedly threatened by Pyongyang, warned that North Korea would face even stiffer sanctions and more isolation if it engaged in any further provocative acts. Speaking through a translator, she said the mere fact the three leaders were huddling to discuss North
Korea carried 'huge significance.'

As a nuclear security summit opened in Washington, the US said a strengthened nuclear security agreement among nations was finally set to take force, including new criminal penalties for smuggling nuclear material.

Recent ratification by a critical mass of countries cleared the way for the changes to take effect in about a month.

Though nuclear terrorism and the Islamic State group top this year's agenda, concerns about North Korea's nuclear weapons program are also commanding focus as the two-day summit gets under way. Those long-simmering concerns have escalated of late following the North's recent nuclear test and rocket launch. China's influence over North Korea will be front and center later in the day when Obama sits down with President Xi Jinping.

The White House said that meeting was also an opportunity for Obama to press US concerns about human rights and China's assertive territorial claims in waters far off its coast.

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