US President Barack Obama warned that extremist "madmen" from the Takfiri group, ISIL, would not hesitate to launch a catastrophic nuclear attack.

US President Barack Obama warned that extremist "madmen" from the Takfiri group, ISIL, would not hesitate to launch a catastrophic nuclear attack.

At a global summit in Washington, Obama hoped to galvanize global action to prevent Takfiris from getting hold of nuclear weapons or material for a "dirty bomb.”

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

He painted an apocalyptic picture of the impact of a nuclear terror attack.

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."

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 ISIL 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.