US President Barack Obama vowed Thursday the US will stand "united" with allies South Korea and Japan in the face of nuclear provocations by North Korea.

Following a meeting with South Korean President Park Geun Hye and Japanese Prime Minister Shinzo Abe at the Nuclear Security Summit in Washington, Obama stressed the importance of "vigilantly" enforcing UN Security Council resolutions on North Korea's nuclear programme.

The ongoing nuclear threat from North Korea and the danger that Islamic State terrorists could acquire a nuclear weapon are on the agenda as world leaders gather Thursday for the fourth and final Nuclear Security Summit of Obama's presidency.

"Trilateral security cooperation is essential to maintaining peace and stability in north-east Asia, deterring the North Korean nuclear threat and the potential of nuclear proliferation as a consequence of North Korean activities," Obama said of cooperation with the key US allies.

The nations agreed to work to outline additional steps to ensure a denuclearized Korean peninsula, ratchet down tensions in the region and alleviate the suffering of the North Korean people in the face of human rights violations by the regime, he said.

Park said the allies agreed to coordinate not only on recent UN Security Council resolutions on North Korea but also on their own national sanctions to increase pressure on Pyongyang.

"I stand here together with the leaders of the United States and Japan, and warn once again that the international community will by no means condone North Korea's provocation, and that should it choose to undertake yet another provocation, it is certain to find itself facing even tougher sanctions and isolation," she said.

The European Union meanwhile expanded its export and import restrictions on North Korean goods on Thursday in line with the Security Council resolution.

Dan Kritenbrink, a top Obama advisor on Asia, said the meeting came as a recognition that the security of the three nations is linked and that they must work together to counter the North Korean threat.

Obama met later Thursday with Chinese President Xi Jinping, and the leaders also stressed the importance of a denuclearized Korean peninsula and the importance of stopping nuclear smuggling.

"Of great importance to both of us is North Korea’s pursuit of nuclear weapons, which threatens the security and stability of the region," Obama said at the outset of his meeting with Xi, adding that they were both committed to "full implementation of UN sanctions" against North Korea.

Tensions have been high on the Korean peninsula since the beginning of the year, when Pyongyang conducted a nuclear test and long-range rocket launch, prompting increased international sanctions.

The gathering is the fourth such summit aimed at improving global nuclear security since US President Barack Obama laid out non-proliferation as a key part of his foreign policy agenda in 2009.

Some 50 heads of state and government and representatives from international organizations will discuss ways to reduce and secure nuclear stockpiles. Discussions will include threats by terrorist groups such as Islamic State to urban areas worldwide.

Deputy National Security Advisor Ben Rhodes noted Thursday that though there has been a significant effort to reduce stockpiles of highly enriched uranium, action must still be taken to reduce nuclear materials, better secure existing materials and prevent it from being transferred.

The recent terrorist attack in Brussels and reports that the attackers had monitored a Belgian nuclear scientist, show that Islamic State has an interest in obtaining nuclear material, Rhodes said, noting the pressing need to increase standards around the world.

On Friday, Obama will hold talks with the members of the five permanent members of the UN Security Council plus Germany to discuss the progress on the nuclear deal reached last year with Iran.

The leaders gathering for the Nuclear Security Summit will meet for dinner Thursday at the White House before the bulk of discussions on Friday with three plenary sessions with leaders and ministers.

The United Nations, International Atomic Energy Agency and Interpol will address the summit over lunch, and the terrorist threat will be on the agenda for Friday afternoon before Obama holds a closing press conference.

Russia, which played a key role in the nuclear deal with Iran, is not participating in the summit.

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