US Secretary of State Condoleezza Rice has pressed Pakistan to tell Washington everything it knows about the AQ Khan nuclear weapons-smuggling network.
The US suspects Pakistan is withholding information
"We all have an interest in knowing how it happened so we can safeguard against this kind of black market" activity, she said on a visit to Pakistan.
AQ Khan, the father of his country's nuclear weapons programme, has admitted passing material to other countries.
Ms Rice also urged Islamabad to stay on the path towards democracy.
The AQ Khan nuclear smuggling network was revealed a year ago and since then, the scientist - once a hero for spearheading the country's nuclear weapons programme - has lived under virtual house arrest.
But the US believes Pakistan has not disclosed everything it knows about his activities.
"We have had co-operation from Pakistan ... but I do not doubt that we all have an interest in knowing what happened," she said in a joint press conference with her Pakistani counterpart, Khurshid Kasuri.
Pakistan's foreign ministry said on Thursday the AQ Khan network had been completely dismantled, Reuters news agency reported.
Time magazine reported last month that the network was still operational, and Pakistan later admitted Dr Khan had given "a few centrifuges" to Iran.
Pakistan is playing a dangerous game with the US, the BBC's Aamer Ahmed Khan says, keeping information to use as a bargaining chip.
Islamabad learned the hard way that once you give powerful allies all they need, they have no more use for you, he says.
Ms Rice also kept up Washington's moderate pressure on Pakistan to allow more democracy.
"We look forward to the evolution of a democratic path towards elections in 2007," she said in Islamabad.
She declined to say whether President Pervez Musharraf should step down as head of the country's armed forces.
He seized power in a military coup in 1999.
She praised "the courage of the Pakistani leadership and the courage of the Pakistani people and the armed forces in the fight against terrorism".
Pakistan has been a key regional ally of Washington, withdrawing support for the Taleban after the 2001 attacks on the US.
Ms Rice is on a trip to South and East Asia that will also take her to Japan, South Korea and China.
She has already visited India and Afghanistan during her eight-day tour.
Earlier in the day, she made her first visit to Afghanistan, when she let slip that the country's parliamentary elections would be delayed.