A Taliban insurgency and the war in neighbouring Afghanistan have put Pakistan's nuclear arsenal at risk giving rise to a "troubling" situation, an arms control expert who served as former US President George W Bush's national security adviser has said.

"The situation in Pakistan is troubling from a lot of perspectives," Stephen Hadley, who now advises Washington-based think-tank the US Institute of Peace said.

"There is a lot of concern about what happens to Pakistan's nuclear weapons if the government fragments in some way," he said on Sunday at an international security conference in Halifax.

Hadley said there was concern in the Bush administration after the September, 2001 terrorist attack that US-led military action inside Afghanistan might destabilise Pakistan and could even lead to a Taliban government.

"So far that hasn't happened, and Pakistan's nuclear weapons remain firmly in the control of the established civilian government," Hadley was quoted as saying by media reports here.

He said the US has assisted Pakistan since 9/11 in maintaining legitimate command and control efforts over its arsenal.

"Whenever we checked with our military and intelligence people, we said, 'Is this a nuclear arsenal at risk?' The answer so far has always been, 'No,'" Hadley said.