Visions of a mushroom cloud over a US city may have led America into a dubious war in Iraq, but the threat of nuclear terror has lost none of its power to fixate US leaders and shape foreign policy.

President Barack Obama put counter proliferation at the center of his political project, earning himself a Nobel Peace Prize, and has worked to secure radioactive material around the globe ever since.

He arrives in Seoul for the second Nuclear Security summit on Sunday in the next step in that quest, though the meeting will be overshadowed by nuclear standoffs with Iran and North Korea.

Views among scientists differ on whether a terror group like Al-Qaeda could build and detonate a primitive nuclear bomb on a US city.

But no president will take the threat lightly after seeing the impact of mass terrorism wreaked by the September 11 attacks in 2001.

Obama said while hosting the first nuclear summit in Washington two years ago that a nuclear strike on a major populated area could change the global security landscape for years to come.

"The ramifications economically, politically and from a security perspective would be devastating," he said.

Analysts say that Obama's concern is justified.

"What we have seen is increasing evidence of intentions... it is not just Al-Qaeda, it is other organisations as well," said Sharon Squassoni, of the Center for Strategic and International Studies.

"It is pretty shocking how much material is out there. 1440 tonnes of highly enriched uranium, 500 tonnes of separated plutonium (which is) weapons ready."

Between 33 and 110 pounds (15 to 50 kilogrammes) of uranium enriched to 90 percent could make a simple nuclear bomb, while 14 pounds (6 kilogrammes) of plutonium would be needed, according to the Union of Concerned Scientists.

Obama's globe trotting has been pared back to a minimum in election year, but his willingness to fly half way around the world to Seoul points to the severity of the nuclear threat.

"You have dozens of nations coming together behind the shared goal of securing nuclear materials around the world, so that they can never fall into the hands of terrorists," said Ben Rhodes, a deputy national security advisor.

Failure, he said, would result in "frankly, ... the gravest national security threat that the American people could face."

Efforts to secure radiological material, in militaries, laboratories or medical establishments are at the center of the broad US agenda with states as diverse as Russia, China, Chile, South Africa, the Ukraine and even ally Canada.

The issue shapes foreign policy -- luring Obama into dialogue with leaders like Kazakh President Nursultan Nazarbayev, despite his poor State Department report card over abuses of political, judicial and press freedoms.

Nazarbayev has earned a meeting with Obama after his cooperation to help secure highly enriched uranium and plutonium with the help of millions of dollars in US taxpayer money.

Analysts say the Obama-led effort to secure nuclear stocks has made progress since the Washington summit -- though there is still some way to go.

"I think America is absolutely safer now than it was three years ago," said Kingston Reif of the Center for Arms Control and Non Proliferation.

"Seven countries have removed all their highly enriched uranium. That is material that is no longer capable of being used by terrorists in some kind of nuclear explosive device."

While America works to secure radioactive stockpiles in the former Soviet Union and elsewhere, senior US officials also worry that a state like Iran or North Korea could pass nuclear materials to a radical group.

Obama deploys one argument against Iran that is strikingly similar to one used by his predecessor George W. Bush to justify the 2003 invasion of Iraq, on the basis of Saddam Hussein's never found weapons of mass destruction.

"There are risks that an Iranian nuclear weapon could fall into the hands of a terrorist organization," he told the US-Israel lobby AIPAC recently.

The showdowns with Iran and North Korea challenge the case that Obama's nuclear agenda, rolled out in a soaring speech in Prague in 2009 is a success.

But experts say, Obama has made some real progress, with 80 percent of commitments made at the Washington summit fulfilled.

He honored a vow to forge a new START treaty with Russia to reduce Cold War nuclear arsenals but is still trying to persuade the Senate to ratify the Comprehensive Test Ban Treaty.

Slow headway is being made meanwhile towards an updated Nuclear Non Proliferation Treaty (NPT).