The security ring surrounding Abdul Qadeer Khan’s sprawling mansion at the foot of Margalla hills in Islamabad has visibly tightened in recent weeks.

Uniformed soldiers with machine guns block all access to the property and scrutinise every passing vehicle.

Mr Khan, the "Father" of Pakistan’s nuclear bomb, has been under house arrest since February 2004 when he confessed to operating an international bazaar for nuclear technology, and spends his days confined with his Dutch wife, Henny, inside his well-protected home.

The iron front gate is completely covered to ensure that no one sees him taking his evening stroll in his lush green garden. Even his daughter, Ayesha Khan, has been stopped from entering the house and his fleet of antique cars languishes unused.

Pakistani military intelligence officials visit him. A long list of others would like to but are forbidden. That list is headed by the Americans, for Mr Khan is perhaps the only man outside Iran who knows precisely what nuclear technology the Tehran regime has received, and what it plans to do with it.

The United States has repeatedly asked the Pakistani authorities for permission to interrogate him, but is always rebuffed. Pakistan recently announced that its investigation into the whole affair was closed.

Ed Royce, the Republican chairman of the congressional sub-committee on non-proliferation, said: "The A.Q. Khan network has been described as ‘the Wal-Mart of private-sector proliferation’. Its handiwork has helped deliver to us two of the most threatening security challenges we face: North Korea and Iran."

Like other congressmen, Mr Royce expressed deep concern that Mr Khan remained "off-limits to foreign investigators" even though Pakistan receives $700 million (£379 million) in aid from the US was designated a significant ally in the War on Terror. "The US and the international community should expect more from Pakistan’s Government," he said.

A senior Pakistani official said that the US continued to press Islamabad for more information on Mr Khan’s network, "but we have told them in no uncertain terms that he is off-limits".

He insisted that Pakistan had shared all it knew with the US and the International Atomic Energy Agency (IAEA), as well as putting questions to Mr Khan on behalf of US investigators. Pakistan should be trusted with the investigation and anything else would be violation of national sovereignty, he added.

Mr Khan has confessed to passing nuclear technology to Iran, Libya and North Korea.

Mahmoud Ahmadinejad, the Iranian President, claimed last month that his country was conducting research on a more advanced P-2 centrifuge. Mr Khan’s deputy, Buhary Seyed Abu Tahir, has told interrogators that the network probably supplied three samples of the P-2 centrifuges, even though Tehran insists it has only received drawings of the machines.

Documents discovered by the IAEA suggest the Khan network might also have given Iran information about how to make the hemispheres of uranium metal needed for nuclear weapons.

But Washington believes the Khan network may still be active. The US Congress has been told how Swiss police recently foiled a plot to ship 60 tons of specialised aluminum tubes — used for building parts of a centrifuge cascade to enrich uranium — through Europe to Pakistan.

According to Andrew Koch, a defence expert, this involved using a middleman in Britain who was not previously known to be a Khan associate. The tubes, which he said could have ultimately been sent to Khan network customers, were eventually seized in the UAE by government authorities.

The US State Department will not comment on the case but a senior Western diplomat said that while there was no formal request for direct access to Mr Khan, the US believed the case was far from over. "We still want to know more about his network. There are many questions which have remained unanswered," he said.

The renewed international attention on Mr Khan has put President Musharraf’s Government in an awkward situation because the scientist is still revered by Pakistanis as a national hero whose birthday is celebrated in mosques and whose portrait hangs in public places.

The Pakistani Senate recently backed unanimously a resolution appreciating the contribution of Mr Khan and his associates in developing the country’s nuclear programme and ruled out handing him over to the Americans.

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