The top US diplomat, who arrived in Islamabad on Wednesday at the start of a three-day visit to the nuclear-armed Muslim nation, said Washington had a "high degree of confidence" that the country's atomic arsenal was safe.

"But we worry about proliferation and we have good reason to worry about proliferation," she said, alluding to the reputed father of Pakistan's nuclear bomb, Abdul Qadeer Khan.

The United States has warned that the disgraced scientist, who was effectively put under a five-year house arrest after he admitted leaking nuclear secrets to Iran, North Korea and Libya, is still a proliferation risk.

"We want to encourage Pakistan to join with us in the non-proliferation review conference that will be held next spring," Clinton told reporters travelling with her.

"We want them to work with us on the Fissile Material Cutoff Treaty. We want them to really understand how serious a threat we face."

She called on Pakistan's military to face up to the potential gravity of the threat in a country Washington has put at the heart of the fight against Al-Qaeda and on the frontline of the war in Afghanistan.

"We know Al-Qaeda and their related extremist allies are always on the hunt for nuclear material and it doesn't have to be a lot to create a very damaging explosion with extraordinary psychological and political ramifications.

"Now that we see that the Pakistani military recognises the threat posed, we want them also to imagine what that threat would be with a nuclear weaponised terrorist group in their midst.

"It's not just about what might happen in our country or in Europe. It's what could happen in Pakistan and what the impact of that would be," she said.

Pakistan, along with presumed atomic powers India, Israel and North Korea, has failed to sign the Nuclear Non-Proliferation Treaty, whose next conference is scheduled for May 2010 in New York.