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

Hoping to galvanize global action to prevent extremists 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.

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."

The summit -- attended by dozens of world leaders and delegates -- is focused on securing global stockpiles of nuclear materials, stored by the military and by the medical and power industries.

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."

- N. Korea concerns -

Though the summit is focused on fissile stockpiles, other nuclear concerns inevitably have drawn broad attention, including North Korea and its continued testing of nuclear devices and ballistic missiles.

The reclusive nation fired another short-range missile off its east coast on Friday, the latest in a series of North Korean missile launches during what has been an extended period of military tension on the Korean peninsula.

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

- New steps -

Halfway through the closing day of the summit, delegates described a series of incremental measures, such as enhanced cooperation between nations.

Obama and Japanese Prime Minister Shinzo Abe announced that Japan had removed all its highly-enriched uranium and separated plutonium fuels ahead of schedule. The fissile material will be "downblended" in the United States for civilian use or eventual disposal.

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 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.

- Shadows of Putin, Trump -

This is the fourth in a series of nuclear security summits convened at Obama's behest and with the president leaving office next year, it may well be the last.

But it risked being overshadowed by two men who were not even there: Republican presidential frontrunner Donald Trump and Russian President Vladimir Putin.

Experts say Putin's refusal to attend has made it almost impossible to achieve substantive reductions in fissile material -- the vast majority of which is held by the militaries of Russia and the United States.

"This nuclear security summit is supposed to address all of the stocks, but truth is that all they address really is a small proportion of civilian stocks," Patricia Lewis, international security research director at British think tank Chatham House, told AFP.

Obama foreign policy advisor Ben Rhodes earlier described the lack of Russian participation as "counterproductive."

America's presidential election also took center stage, with questions about Trump's suggestion that Asian allies should develop nuclear weapons.

"It would be catastrophic for the United States to shift its position and indicate that we somehow support the proliferation of nuclear weapons," Rhodes said.