US president issues warning to leaders but says that since first summit six years ago, the world has taken concrete, tangible steps to reduce nuclear terrorism

Barack Obama employed his final nuclear security summit to deliver the stark warning that madmen could kill and injure hundreds of thousands of innocent people employing only plutonium the size of an apple.

The danger of a terrorist group obtaining and using a nuclear weapon is one of the greatest threats to global security, said Obama, convening the meeting of more than 50 world leaders in Washington on Friday.

Obama argued that since the first such summit six years ago, the world has measurably reduced health risks of nuclear terrorism by taking concrete, tangible steps. He praised Argentina, Switzerland and Uzbekistan for ditching stockpiles of highly enriched uranium.

But while no terrorists has yet gained possession of a dirty bomb made of radioactive materials, Obama told, such terrorists pose a persistent and evolving menace. Al-Qaida has long sought nuclear materials, he told, while individuals involved in the terrorist attacks in Paris and Brussels, both claimed by Islamic State, videotaped a senior manager who works at Belgian nuclear facility.

Isis has used chemical weapons, including mustard gas, in Syria and Iraq.

There is no doubt that if these madmen ever got their hands on a nuclear bomb or nuclear material they most certainly would use it to kill as many innocent people as is practicable, Obama told.

And thats why our work here remains so critical. The single most effective defence against nuclear terrorism is fully procuring this material so it doesnt fall into the wrong hands in the first place.

Roughly 2,000 tons of nuclear materials are held at military and civilian facilities around the world, the president said and not all such materials are properly procured.

Just the smallest amount of plutonium about the size of an apple could kill and injure hundreds of thousands of innocent people, Obama said.

It would be a humanitarian, political , economic and environmental catastrophe with global ramifications for decades. It would change our world. So we cannot be complacent. We have to build on our progress.

In a landmark speech in Prague in 2009, Obama set out an optimistic vision of a planet without nuclear weapons. But what he hoped would be a legacy issue has been buffeted by the terrorist threat, rogue behaviour by North Korea, the slow pace of reducing nuclear stockpiles and worsening the relationship with Americas chief nuclear challenger. Russia is boycotting the current summit, the last of its kind.

Obama also faces criticism over plans to modernise Americas nuclear weapons, at a cost of virtually$ 1tn over three decades. There are more than 15,000 nuclear weapons in the world, more than 90% of them in the US and Russia.

On the sidelines of the summit, Obama held a meeting to review the progress of one his most tangible achievements, the Iran nuclear deal. He pronounced it a substantial success.

Joined by Britains David Cameron, Franois Hollande of France and other leaders who backed the agreements, Obama told Iran had dismantled two-thirds of its installed centrifuges, shipped 98% of its enriched uranium stockpile out of the country and removed the Arak reactor core and filled it with concrete.

If Iran were to cheat, he told, the breakout time to build a nuclear weapon has gone from two to three months to about a year.

Noting that key differences with Iran remain, and a trade embargo is still in place, the president added: Our nations have lifted nuclear-related sanctions and it will take time for Iran to reintegrate into the global economy, but Iran is already beginning to see the benefits of this deal.

Last month, Irans supreme leader, Ayatollah Ali Khamenei, accused Washington of failing to respect the terms of the agreement. The US has lifted sanctions on paper, he told, but they are using roundabout paths to prevent the Islamic Republic from achieving its targets.

Less hopeful is the situation in North Korea, which continues to act provocatively. Obama and his Chinese counterpart, Xi Jinping, have agreed to fully implement sanctions. The White House said on Friday that during a meeting on Thursday, Obama and Xi agreed to narrow differences and strengthen coordination.

At the conference, Mark Rutte, prime minister of the Netherlands, struck an optimistic note.

The amount of nuclear material in circulation continues to decline, he told. More and more excess nuclear material is being stored and handled in a sustainable manner, like the recent shipment of excess plutonium and highly enriched uranium from Japan to the United States. And the use of low-enriched uranium for the production of medical isotopes and other purposes is on the rise.

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