Pakistan admitted last week that Khan had supplied Iran with centrifuges, used to enrich uranium for atomic warheads, but it said the government was not involved and has refused to give him up for questioning by other countries.

"It is a network that we want to make certain that its tentacles are broken up as well," Rice told a press conference after talks with Pakistani leaders.

She said the United States has had cooperation from Pakistan to try to ensure the network is broken up and to get as much information as possible.

"I do not doubt that we all have an interest in knowing what happened, that we all have (an) interest in making sure that this network cannot... continue to operate in any way," she told a joint press conference with her Pakistani counterpart Khurshid Kasuri.

Khan, who is considered a national hero in Pakistan and is the father of its nuclear bomb, confessed in February 2004 to leaking secrets to Iran, North Korea and Libya after a government probe into nuclear proliferation.

Khan was later pardoned by President Pervez Musharraf but has been living under virtual house arrest in Islamabad.

Rice, who is on her first visit to the region since assuming office in January, said Washington still wanted to know how the network was able to operate.

"Most importantly we all have an interest in knowing how it happened, so that we can safeguard against this kind of black market entrepreneurship in the future," she said.

Pakistan has repeatedly said it would not allow any foreign country or agency to interrogate the nuclear scientist, despite its admission that Khan supplied Iran with centrifuges.

Washington believes the technology has enabled Iran to enrich uranium to a level required for making nuclear weapons.

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