Khan, the so-called father of Pakistan’s nuclear programme, is known to have helped Iran, Libya and North Korea to acquire centrifuges and other nuclear components on the black market, but American intelligence sources believe his net was cast wider.

A hero in his own country, Khan was sacked as President Pervez Musharraf’s special scientific adviser this year after admitting that he had passed on nuclear secrets to other countries.

John Bolton, the US undersecretary of state for arms control, has voiced fears that Khan had “several other” customers seeking to acquire the bomb. “There is more out there than we can discuss publicly,” he told the United Nations in April. “It’s one of the reasons why the depth of our concern about the international black market in weapons of mass destruction is as substantial as it is.”

Danielle Pletka, an expert on Middle Eastern nuclear proliferation at the American Enterprise Institute, said: “There’s very wide suspicion that Syria was part of the A Q Khan network.” The scientist visited Syria in the late 1990s and is thought to have met Syrian officials secretly in Iran.

Pletka claims the scientist cut a deal with Musharraf, limiting his confession to the three countries that were already proven to be part of his nuclear sales ring while avoiding mention of three other interested parties: Syria, Egypt and Saudi Arabia.

“The Syrians have long had a chemical weapons programme but to the best of people’s knowledge never had a nuclear programme, but there is mounting evidence that this is no longer the case,” Pletka said.

The CIA reported to Congress last year that it viewed “Syrian nuclear intentions with growing concern”. Recent intelligence intercepts suggest that Syria not only might have acquired centrifuges, but might be operating them.

Opinion is divided in the intelligence community as to the extent of the threat. The American State Department said: “The United States government has consistently outlined our concerns with regard to Syria’s pursuit of weapons of mass destruction. We are very interested to learn the scope of the A Q Khan network, but we are not in a position to say with certainty that Syria has centrifuges.”

Bolton has been frustrated by limited co-operation from Khan and his associates. “If part of that network is exposed, you don’t really know whether you’ve exposed all of it or not, or brought it down,” he said earlier this year.

Tensions with Syria have been ratcheted up by the conflict in Iraq. There are concerns that the Syrians are letting fighters, terrorists and weapons cross its borders and the prospect, however distant, of a nuclear-armed Ba’athist state is particularly chilling to Americans. The acquisition of centrifuges would be an important step towards obtaining the weapons-grade material needed to develop a nuclear bomb.

“It’s no secret the Syrians have historically sought an answer to Israel’s overwhelming conventional superiority and have an active biological and chemical weapons programme,” said Steven Cook, of the Council on Foreign Relations. “If they could acquire a nuclear option it would shift the strategic situation in the region.”

Cook doubts, however, that Syria has the capacity and infrastructure to produce a nuclear bomb, even if it has acquired some of the technology. “They’d still be looking to buy missiles,” he said.

President George Bush imposed trade sanctions on Syria last month, claiming that its actions posed an “unusual and extraordinary threat to the national security, foreign policy and economy” of America. He accused Syria of pursuing weapons of mass destruction and missiles, in particular advancedchemical weapons capabilities.

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