WASHINGTON, Sept 4: Information received from Pakistan has allowed US investigators to uproot the network that had placed the nuclear technology in the market, US officials and lawmakers say.

Pakistan's contribution has "pinned down, intelligence-wise, a great deal we did not know before," says Senator Richard Lugar, Chairman of the Senate Foreign Relations Committee.

In a recent speech, the Republican senator rejected allegations that Islamabad was not sufficiently active or cooperative in investigating the activities of nuclear proliferators called the Khan network by the international media.

Pakistan's cooperation, he said, had enabled US investigators to track "suspicious activities" in North Korea, Iran and Libya.

According to US officials, Pakistan had been assisting Washington's efforts to unravel the Khan network long before Dr A.Q. Khan's confession in February that he had supplied nuclear technology to these three countries.

Privately, US officials say that the United States was particularly concerned about a particular model of the centrifuge used for enriching uranium, called P-2 or Pakistan-2. This is a compact model, which occupies a relatively smaller space and can avoid detection because of its compactness. Several such centrifuges were discovered in Iran.

But US investigators were not only worried about those who had received nuclear technology from the Khan network. They were also concerned about the existence of a network of suppliers that enabled Pakistan to build up its own nuclear programme against the wishes of the United States and other Western powers.

Despite excessive restrictions aimed at preventing Pakistan from acquiring nuclear technology, Pakistan was able to buy from this network whatever it needed to build a weapon-grade, sophisticated nuclear plant.

"The Americans feared that others could use the same route to make nuclear weapons and wanted to break up this network and they knew that they could not do so without Pakistan's cooperation," said an official source who did not want to be identified.

The network spread over the Middle East, Western Europe and some former Soviet states and with Pakistan's help, US security agencies have uprooted this network too, the source said.

"This has made it extremely difficult for another country or group to use the sources that Pakistan did to fulfil its nuclear ambitions," the source said.