For the first time, serious indicators have emerged that two Pakistani nuclear scientists were helping Al Qaeda build nuclear weapons, according to a top American expert.

"Al Qaeda wanted the Pakistani scientists' help in making radiological dispersal devices... [Pakistani nuclear scientist Sultan Bashir-ud-din] Mahmood and his colleagues appear to have provided Al Qaeda a road map to building nuclear weapons," David Albright, President of Washington-based Institute for Science and International Security, said in a recent paper.

He, along with another ISIS research analyst Holly Higgins, said the move was 'typically very helpful in understanding the steps that must be accomplished in making a nuclear weapon, identifying the necessary equipment and technology, and locating suppliers of key equipment'.

"In additions colleagues appear to have recruited other scientists with more direct knowledge of making nuclear weapons," they said.

Two nuclear scientists, Mahmood and Chaud who had founded 'Ummah Tameer-e-Nau' allegedly for relief and development work in Afghanistan, had been in close touch with Osama bin Laden and other Al Qaeda leaders.

The duo were arrested and interrogated by the Federal Bureau of Investigation and the Pakistani police, but released on conditions that they would remain under government control, submit to travel restrictions and limit their communications.

"A surprising piece of information was UTN's interest in uranium mining. It has been known for long that Afghanistan has uranium resources. But that Pakistani nuclear scientists were planning to extract uranium, increases suspicions about their intentions," Albright and Higgins said in their paper.

Stating that findings of uranium in Afghanistan would have made any weapons programme more indigenous, they said fortunately, the fall of the Taliban regime ended the threat.

Apart from working on Pakistan's secret gas centrifuge programme which ultimately produced the highly-enriched uranium used in its nuclear weapons, Mahmood had also designed the Khushab reactor, 'an unsafe guarded reactor project that depended extensively on illicit procurement from abroad', the American experts said.