Pakistan's nuclear weapons programme is expanding at a rapid pace and its arsenal could overtake that of France, which ranks fourth in the world in possession of nuclear weapons.
The US, Russia and China have bigger stockpiles than France. India ranks sixth.According to new satellite imagery obtained by Newsweek magazine, Pakistan is "aggressively accelerating construction" at the Khushab nuclear site, 225 km south of Islamabad.
The images prove Pakistan will soon have a fourth operational reactor, "greatly expanding plutonium production for its nuclear weapons programme".
The White House declined comment but a senior US congressional official told Newsweek that intelligence estimates suggest Pakistan had already developed enough fissile material, or bomb fuel, to produce over 100 warheads and manufacture between eight and 20 weapons a year.
"There's no question it's the fastest growing programme in the world," the official said.
"The build-up is remarkable," said Paul Brannan of the Institute for Science and International Security.
"And that nobody in the US or in the Pakistani government says anything about this is perplexing."
Eric Edelman, under-secretary of defence in the George W Bush administration, said: "You're talking about Pakistan even potentially passing France at some point. That's extraordinary."
Pakistani officials said the build-up was a "response to the threat from India".
The report said it was dangerous that Pakistan was stockpiling fissile material.
"The amount is what's so alarming," said Mansoor Ijaz, who has played a role in back-channel diplomacy between Islamabad and New Delhi.