The nightmare scenario of a mushroom cloud over a major city has led US President Barack Obama to declare "nuclear terrorism" as the ultimate threat.

But experts are divided over the nature of the threat, with skeptics questioning the likelihood that extremists could manage to acquire, build or detonate a nuclear device.

"A 10-kiloton nuclear bomb detonated in Times Square could kill a million people," Secretary of State Hillary Clinton said on Friday, making the case for urgent international action.

"Beyond the human cost, a nuclear terrorist attack would also touch off a tsunami of social and economic consequences across our country," she said.

At a summit in Washington next week, Obama plans to press leaders from around the world to take concrete steps to secure nuclear material before it falls into the wrong hands.

Administration officials describe their effort as a race against time to "lock down" weapons-grade uranium and nuclear warheads before terror groups can steal or buy them.

Extremists could build an improvised nuclear device with as little as 25 kilograms (55 pounds) of highly enriched uranium, and experts say hundreds of sites holding atomic bombs or material may lack adequate security.

Since 1993, there have been at least 15 cases of smuggling involving weapons-grade nuclear materials, according to the International Atomic Energy Agency.

Western officials worry about unrest in nuclear-armed Pakistan allowing Al-Qaeda to get a hold of an atomic weapon, or Iran's nuclear ambitions leading eventually to the arming of Hezbollah or Hamas with nuclear weapons.

North Korea has already been accused of selling nuclear technology to Syria, and officials worry financial desperation could prompt the regime to do business with terror groups.

The possibility of extremists building a working atomic bomb on their own remains remote, most experts said.

But Al-Qaeda and other terror groups are clearly interested in acquiring nuclear technology, a US counter-terrorism official told AFP.

"Developing a nuclear device involves a highly sophisticated technical process, and Al-Qaeda doesn't seem to have mastered it based on what we know now," said the official, who spoke on condition of anonymity.

Although Al-Qaeda has failed so far in its bid to acquire the bomb, the group has skillfully promoted the idea that it might one day launch a nuclear attack, said Brian Jenkins, a senior adviser at the RAND Corporation, a California-based think tank.

"Al-Qaeda appears to have figured out that fomenting nuclear terror does not require possession of nuclear weapons at all," Jenkins said.

"Their communication has been so successful on this issue that Al-Qaeda has become the world first virtual terrorist nuclear power without possessing nuclear weapons," said Jenkins, author of "Will Terrorists Go Nuclear?"

A former CIA officer who led efforts to track weapons of mass destruction, Rolf Mowatt-Larssen, said the probability of an Al-Qaeda nuclear attack was relatively low, but the group's goal was clear.

"There's a specific nuclear intent," he said, adding the network aspires to stage assaults "to change the world, to do something spectacular."

The failure to find WMD in Iraq damaged the US government's credibility on the issue. But intelligence on Al-Qaeda's pursuit of nuclear weapons was more detailed and more reliable, according to Mowatt-Larssen.

US officials say preventing a nuclear attack by extremists is entirely possible -- provided governments move to safeguard the world's supply of highly enriched uranium and plutonium.

Obama's deputies argue a newly agreed arms control deal with Russia will help bolster Washington's credibility as it lobbies other countries to help stop the spread of nuclear weapons and equipment.

The scaling back of US and Russian nuclear arsenals under the arms control agreement could, however, increase the risk of a catastrophic attack.

"As you decommission these weapons, you take them out of the military inventory," Jenkins said.

Plutonium and uranium end up in "a huge stockpile of fissile material which is in fact more difficult to protect than a single nuclear weapon."