Pakistan may have tested a nuclear weapon in partnership with North Korea, the New York Times indicated on Friday.
The revelations about international nuclear trading by Pakistani scientist Abdul Qadeer Khan have rekindled a debate in the American intelligence community over this unresolved but crucial strategic question from the last decade, it said.
Clues were detected after underground tests that Pakistan carried out in May 1998, when it proved to the world that its own efforts to build nuclear weapons had succeeded, the report said.
The report quoted former and current American intelligence officials as saying that a jet sent to sample the air after the final test in the wastelands of the Baluchistan desert picked up traces of plutonium.
That surprised experts at the Los Alamos national laboratory in the US, because Pakistan said openly that all of its bombs were fuelled by highly enriched uranium, produced at Dr Khan's laboratories.
Among the possible explanations debated after the tests, the report said, was that North Korea, perhaps in return for help from Khan, might have given Pakistan some of its precious supply of plutonium to jointly test an atomic weapon.
Also, the last test was about 100km away from the first test site. The shaft leading to the bomb was dug vertically rather than horizontally, a lower cost method, the report said.
The detonation was also smaller. Pakistani officials said they had used a 'miniaturised' device, but gave no other details. By all accounts, Khan was closely involved with that final test, the paper said.
It takes far less plutonium to make a large nuclear explosion, so plutonium missile warheads are smaller and more powerful.
The Central Intelligence Agency, the report said, has been urgently preparing a report this week on what North Korea may have gained from Khan's nuclear dealings to supply new evidence to American negotiators in the Beijing talks.
A day after the test a reporter asked Khan about rumours that Pakistan had once tested a weapon in China. He said, "No country allows another country to explode a weapon."
But the report said that at the Los Alamos laboratory, some experts believed that might have been exactly what happened. Pakistan, most analysts believed, had insufficient material and experience to make a plutonium bomb.
"It could only have come from one of two places: China or North Korea," said one senior intelligence official adding, "it seems like China had nothing to gain."
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