US President Barack Obama said Friday that "significant and meaningful" progress has been made in the last six years in reducing and securing nuclear material around the world.

Speaking at the conclusion of the two-day Nuclear Security Summit, Obama noted that entire continent of South America was free of highly enriched nuclear material, with central Europe and south-east Asia soon to follow.

In a news conference following the summit, Obama called it a "remarkable achievement."

More than a dozen countries have removed all of their highly enriched plutonium and uranium, Obama said. Japan is currently removing a half ton of highly enriched nuclear material, the largest such project in history.

Obama said that the nuclear arsenals of the US and Russia - the largest possessors of nuclear arms in the world - are their smallest in six decades.

He said that continued international focus should be placed on the nuclear behaviour of India and Pakistan, while noting that North Korea posed the most immediate concern.

Much of the summit's second day was devoted to terrorism and the danger of nuclear weapons falling into the wrong hands. The global community amid current terrorist threats "simply cannot afford" not to share "critical intelligence," Obama said.

He told the assembled leaders that the "good news" was that the Islamic State organization is being "squeezed" in Syria and Iraq and losing territory and leadership, while the flow of foreign fighters to the region has been slowed.

Still, the US president said "the threat from foreign fighters returning to commit acts of horrific violence remains all too real."

Given the pressure on the movement, Islamic State members are "lashing out elsewhere," Obama said, noting recent attacks in Turkey and Brussels.

"This means that the sense of urgency that we've shown in destroying [Islamic State] in Iraq and Syria also has to infuse our efforts to prevent attacks around the world," he said.

"Looking around this room, I see nations that represent the overwhelming majority of humanity - from different regions, races, religions, cultures," Obama said, speaking to leaders representing some 50 countries. "But our people do share common aspirations to live in security and peace and to be free from fear."

He warned of the danger of nuclear weapons falling into the hands of the Islamic State group: "There's 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 possible."

Noting that "no terrorist group has succeeded thus far in obtaining a nuclear weapon" and that international efforts to date had "measurably reduced the risks" of nuclear material falling into the wrong hands, Obama pointed out the threat nevertheless "persists."

Al-Qaeda had sought nuclear material, and the attackers behind last week's Brussels bombings had videotaped nuclear plant employees, Obama said. He pointed out that Islamic State forces have used mustard gas.

Obama said there was still 2,000 tonnes of nuclear material around the world, and "not all of this is properly secured." Just a small amount - "the size of an apple" - could kill and injure hundreds of thousands of people, he warned.

Russia chose not to attend the summit.

Earlier Friday, Obama met with leaders of the P5+1 group, which negotiated the Iran nuclear deal. He thanked all the nations who participated in bringing the agreement to fruition.

"Thanks to this deal, we have seen real progress," Obama said.

He noted that it will "take time for Iran to reintegrate into the global economy, but Iran is already beginning to see the benefits of this deal."

At the later press conference, Obama suggested that while Iran has followed the letter of the nuclear agreement, its actions sometimes ran counter to its spirit.

Iran must demonstrate to investors that the country is "a safe place to do business," he said.

Obama pointed to a "range of provocative actions" by Iran, such as ballistic missile tests with anti-Israeli slogans, and missile shipments to Hezbollah, which make potential investors "nervous."

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