A nuclear or biological attack by terrorists on the United States is the country's greatest threat, but that is unlikely to happen, experts said.

In a recent CNN interview, U.S. Secretary of State Hillary Clinton said weapons of mass destruction in the hands of a militant group like al-Qaida were the biggest threat to the UnitedStates.

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

But while the threat is a horrific one, it is also one of the least likely to occur, experts said.

For one thing, it is difficult for a non-state entity like al-Qaida -- and most countries, for that matter -- to build a deployable nuclear device, said Scott Stewart, vice president of tactical intelligence at global intelligence company Stratfor.

Some states have access to universities, teams of scientists, huge facilities and large state budgets all aimed at creating a nuclear weapon, and even still those countries have difficulty in producing a usable weapon, he said.

Purchasing a weapon of mass destruction would also be extremelydifficult for militant groups, 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, he said.

And if nuclear weapons-grade material were on the market, a number of nations would scramble to get their hands on it, creating heated competition, he said.

As for U.S. attempts to protect the country from such a threat, the U.S. Federal Bureau of Investigation and the Central Intelligence Agency have placed a high priority on the issue.

"It's hard to determine if they are doing enough," he said. "There's only so much realistically you can do and always more could be done, but they have placed a great deal of resources into it."

Still, that didn't prevent Vice President Joe Biden and former Vice President Dick Cheney from sparring on Sunday over whether or not the Obama administration is doing enough to combat terrorists.

Biden appeared on CBS and NBC in response to Cheney's criticisms of the Obama administration's stance on terror on ABC's" This Week."

"It's the mind-set that concerns me," Cheney said, taking a jab at Obama's handling of terrorism and national security, as well as at the administration's decision to try terror suspects in federal courts instead of military tribunals.

In an interview on CBS' "Face the Nation," Biden said terrorists were unlikely to pull off another 9/11 style attack.

"The reason it's unlikely is because we have been relentless, absolutely relentless in isolating al-Qaida, central al-Qaida, Osama Bin Laden, al-Qaida coming out of the Afghan-Pakistan region," he said.

Speaking on "This Week," Cheney countered that Biden was "dead wrong." "I think, in fact, the situation with respect to al-Qaida to say that, you know, that was a big attack we had on 9/11, but it's not likely again, I just think that's dead wrong. I think the biggest strategic threat the United States faces today is the possibility of another 9/11 with a nuclear weapon or a biological agent of some kind, and I think al-Qaida is out there even as we meet trying to figure out how to do that."

"You don't want the vice president of the United States running around saying, 'Oh, it's not likely to happen,'" he said.

"This is all politics," Stewart said, adding that there were a few differences between the current and former administrations in terms of anti-terror policies, although other experts had different views.

Cheney pointed to a number of Bush policies that the Obama administration has kept in place, such as the use of military tribunals to try terror suspects and the detention facility at Guantanamo Bay, Cuba.

Scott Payne, senior policy advisor at Third Way, a Washington think tank, said that while Cheney had repeatedly accused Obama of being soft on terror, the facts did not back up those claims. Cheney has often said Obama does not think the United States is in a war against terror, despite the Obama administration's references to the "war on al-Qaida."

The Obama administration is also focused on a treaty with Russia that aims to slash nuclear stockpiles, whereas the Bush administration's treaty only shelved nukes, making them more vulnerable to theft by militants, he noted.

Biden made a fair point by saying that another 9/11 scale attack was unlikely, although keeping weapons of mass destruction out of the hands of anti-U.S. radicals was the most pressing national security issue for the United States.

"Even though it's a small chance, the potential devastation is something that you absolutely have to prevent from happening," he said.