US President Barack Obama has warned of a persistent threat of terrorists getting their hands on nuclear materials despite progress in reducing such risks, and called on world leaders to do more to safeguard nuclear facilities.

“There is no doubt that if these madmen ever got their hands on a nuclear bomb or nuclear material, they would certainly use it to kill as many people as possible,” he told a global nuclear security summit in Washington on Friday.

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Obama cited concerns about groups such as al-Qaeda and Islamic State trying to obtain nuclear materials, saying this was no time for the international community to be complacent.

Obama was hosting more than 50 world leaders for his fourth and final summit focused on efforts to lock down vulnerable atomic materials to prevent nuclear terrorism. North Korea’s nuclear defiance was also high on the agenda.

He has less than 10 months left in office to follow through on one of his signature foreign policy initiatives. While progress has been made, many arms-control advocates say the diplomatic process – which Obama conceived and championed – has lost momentum and could slow even further once he leaves the White House in January.

A boycott by Russian President Vladimir Putin, unwilling to join in a US-dominated gathering at a time of increased tensions between Washington and Moscow over Ukraine and Syria, adds to doubts that the meeting will yield any major decisions.

Deadly militant bomb attacks in Brussels last month have fuelled concern that Islamic State could eventually target nuclear plants, steal material and develop radioactive “dirty bombs”.

As official summit meetings began, Obama insisted that “we’ve made significant progress” and said the required 102 countries had ratified an amendment to a nuclear security treaty that would tighten protections against nuclear theft and smuggling.

“Our nations have made it harder for terrorists to get their hands on nuclear materials. We have measurably reduced the risks,” Obama said.saying this was no time for the international community to be complacent.

Despite significant strides by Obama in persuading dozens of countries to rid themselves of bomb-making materials or reduce and safeguard stockpiles, much of the world’s plutonium and enriched uranium remains vulnerable to theft.

Earlier on Friday, Obama convened a separate meeting of the world powers that negotiated a landmark nuclear pact with Iran last July, a critical component of his nuclear disarmament agenda and a major piece of his foreign policy legacy.

He said efforts to implement the deal, which required Tehran to curb its nuclear program in return for sanctions relief, had shown “real progress” but it would take time for Iran to reintegrate into the global economy.

Obama inaugurated the first Nuclear Security Summit nearly six years ago, after a landmark speech in Prague in 2009 laying out the lofty goal of a world free of nuclear weapons.

When Obama leaves office in January, there is no guarantee that his successor, who will be elected in November, will keep the issue a high priority.