While the administration of U.S. President Barack Obama has named nuclear terrorism a major threat to the United States, experts differ over the likelihood of a nuclear attack on U.S. soil.

The matter will see much debate next week, as Obama is expected to host the Global Nuclear Security Summit in Washington in a bid to find international consensus on how to secure vulnerable nuclear materials worldwide and keep them out of the hands of groups such as al-Qaeda.

Some experts paint the threat as dire, pointing out that the know-how to build a weapon and the materials are out there, and contending that that is why Obama is taking the possibility of attack very seriously.

Others downplay the threat, arguing that while an attack would be devastating, the odds of radicals obtaining nuclear materials, constructing a bomb and setting it off are low.

For one thing, building a deployable nuclear device is difficult even for states, in spite of access to universities, teams of scientists, huge facilities and large state budgets all aimed at creating a nuclear weapon. A terror organization not backed by a national government and without access to the full panoply of resources, would be hard pressed to launch a successful nuclear attack, some experts believe.

Purchasing a weapon of mass destruction would also pose a hurdle to militants, some experts said, as the United States spends hefty sums -- around 1 billion dollars per year -- to track and buy fissile material in a bid to keep it off the market.

Still, U.S. Secretary of State Hillary Clinton recently told CNN that weapons of mass destruction in the hands of a militant group such as al-Qaeda were the biggest threat to the United States.

"The biggest nightmare that many of us have is that one of these terrorist member organizations within this syndicate of terror will get their hands on a weapon of mass destruction," she said.

Clinton said al-Qaeda remained "unfortunately a very committed, clever, diabolical group of terrorists who are always looking for weaknesses and openings."

Indeed, the possibility of attack by a "dirty bomb" -- a crude radiological weapon -- came to light in 2002 after the arrest of Jose Padilla, a U.S. citizen accused of plotting an attack on the United States, although the case was dropped and Padilla was instead convicted of conspiring to kill people in an overseas jihad.

In November 2006, British intelligence warned that al-Qaeda was plotting to mount a nuclear attack against British cities by acquiring chemical, biological, radiological or nuclear weapons.

In June 2007, the U.S. Federal Bureau of Investigation released the name Adnan Gulshair Muhammad el Shukrijumah, the alleged al- Qaeda operative said to be in charge of developing plans to simultaneously launch attacks against several U.S. cities.

The Fissile Materials Working Group, an organization collaborating in a series of meetings designed to create consensus on controlling fissile materials, called nuclear terrorism the number one threat the international community faces. And with enough materials worldwide to build 120,000 nuclear bombs, the possibility that a terrorist network could buy or steal such material is far too high, as not all the world's nuclear materials are secure, the organization said in a statement.

Worldwide there are roughly 1,600 metric tons of highly enriched uranium and 500 metric tons of plutonium stockpiled and in weapons. The majority of these materials are in weapons, but some are also used in civil applications, the group said.

The largest stockpiles are in Russia and the second largest is in the United States. The United Kingdom, France and China also have significant stockpiles of highly enriched uranium and the United Kingdom, France and Japan have considerable stockpiles of plutonium both for civil and military (excluding Japan) purposes, the group said.

There are now more than 400 commercial nuclear power reactors operating in 30 countries, which provide 15 percent of the world's electricity, the group said.

Experts said militants need not break into a facility to obtain nuclear materials, as theft could be carried out by an employee of a facility that uses them.