"In terms of a country, obviously a nuclear-armed country like North Korea or Iran pose both a real or a potential threat," Clinton told CNN's "State of the Union", making it clear the Iranians don't yet possess an atomic weapon.

"But I think that most of us believe the greater threats are the trans-national non-state networks," she said, referring to Al-Qaeda and its affiliates in Afghanistan, North Africa, Pakistan, Saudi Arabia and Yemen.

Clinton voiced concerns about Al-Qaeda's level of "connectivity" and warned that Osama bin Laden's followers were increasing the sophistication of the attacks they were planning.

While Al-Qaeda was not getting any stronger and its capacity had been "degraded" in Afghanistan and Pakistan, Clinton cautioned the terror network was evolving to become "more creative, more flexible, more agile.

"They are unfortunately a very committed, clever, diabolical group of terrorists who are always looking for weaknesses and openings and we just have to stay alert."

The U.S. city of Detroit had a narrow escape on Christmas Day when a young Nigerian claiming allegiance to Al-Qaeda, Umar Farouk Abdulmutallab, botched an attempt to bring down a packed transatlantic airliner as it began its descent.

"The biggest nightmare that any 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," Clinton said.

She gave the interview before Iranian President Mahmoud Ahmadinejad ordered Iran's atomic chief on Sunday to begin higher uranium enrichment, raising the stakes in its long-running dispute with the West over its nuclear ambitions.

Clinton said it was "subject to some debate" how close Iran was to getting the bomb but suggested Tehran was purposefully stalling on what she described as a "very reasonable offer" from the international community.