Extremist "madmen" from the Islamic State group would not hesitate to launch a catastrophic nuclear attack, US President Barack Obama warned at a global summit in Washington on Friday.

Hoping to galvanize global action to prevent jihadists from getting hold of nuclear weapons or material for a "dirty bomb," Obama painted an apocalyptic picture of the impact of a nuclear terror attack.

Obama, who is leaves office next January, was hosting a fourth and likely final leaders' summit aimed at reducing the risk of a nuclear holocaust.

The first summit was held in Washington six years ago at Obama's behest, when the young president, fresh from winning the Nobel Peace Prize sketched out a vision of a world without nuclear weapons.

Today he stands as a president on his way out, trying to complete as much of his agenda as possible while Republican front runner Donald Trump garners attention with unorthodox calls for South Korea and Japan to be nuclear armed.

Such utterances, Obama said, "tell us the person who made the statements doesn't know much about foreign policy or nuclear policy or the Korean peninsula or the world generally."

READ: Obama says Trump doesn't know much about foreign policy

But Obama nodded at his own failures too, saying poor relations with Russia -- the world's other major hoarder of nukes -- had hobbled his hopes to reduce conventional nuclear stockpiles.

In a characteristic power play, Russian President Vladimir Putin had pointedly boycotted the summit.

"My preference would be to bring down further our nuclear arsenal," said Obama, adding that he had approached Putin in the hopes of negotiating another arms reduction treaty.

"Because of the vision that he's been pursuing of emphasizing military might over development inside of Russia and diversifying the economy, we have not seen the kind of progress that I would have hoped for with Russia."

Instead, Obama used the summit to push for technical measures to safeguard fissile materials and limit the civilian use of the most dangerous uranium and plutonium.

He also shifted the focus on to North Korea's provocative nuclear tests, the recently agreed nuclear deal on Iran and above all the threat from the Islamic State group.

That threat has loomed large over the two-day summit, amid revelations that the Islamic State group carried out video surveillance on a top Belgian nuclear scientist.

"ISIL has already used chemical weapons, including mustard gas, in Syria and Iraq," Obama said, using an acronym for the Islamic State group.

"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 continue to kill as many innocent people as possible."

Obama said about 2,000 tons of nuclear materials are stored around the world at civilian and military facilities, but some of them are not properly secured.

"Just the smallest amount of plutonium -- about the size of an apple -- would kill and injure hundreds of thousands of innocent people," he said.

"It would be a humanitarian, political, economic and environmental catastrophe with global ramifications for decades," he added.

"It would change our world."

The nuclear security summit comes in the wake of attacks in Paris and Brussels that have killed dozens and exposed Europe's inability to thwart destabilizing attacks or track Islamic State operatives returning from Iraq and Syria.

"As ISIL is squeezed in Syria and Iraq, we can anticipate it lashing out elsewhere," he said.

"We need to do even more to prevent the flow of foreign terrorist fighters."


Bellicose hermit

North Korea's decision to carry out the latest in a series of missile launches during the summit, drew yet more attention to its continued testing of nuclear devices and ballistic missiles.

The summit opened Thursday with Obama trying to forge consensus among East Asian leaders on how to respond to Pyongyang.

Obama also used the summit as a chance to hold "candid" talks with his Chinese counterpart Xi Jinping over Beijing's alleged military buildup in the South China Sea.

US officials have expressed concern that China's actions in the South China Sea are inconsistent with Xi's pledge at the White House last year not to pursue militarization of the hotly contested and strategically vital waterway.

China has argued that the pledge was narrowly focused on one portion of the contested waters.

China claims virtually all the South China Sea despite conflicting claims by Brunei, Malaysia, Vietnam, Taiwan and the Philippines, and has built up artificial islands in the area in recent months, including some with airstrips.