According to new commercial satellite imagery obtained exclusively by Newsweek magazine, Pakistan is "aggressively accelerating construction" at the Khushab nuclear site, about 140 miles south of Islamabad.
The report said the fourth nuclear reactor at Khushab could come online as early as 2013.
The images prove Pakistan will soon have a fourth operational reactor, "greatly expanding plutonium production for its nuclear weapons programme", analysts told the magazine.
The development comes at a time of "unprecedented misgiving between Washington and Islamabad" in the wake of the US raid that killed Osama bin Laden.
The White House declined to comment but a senior US congressional official working on nuclear issues told Newsweek that intelligence estimates suggest Pakistan has already developed enough fissile material 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 especially in this day and age is perplexing."
Unlike Iran, which has yet to produce highly enriched uranium, or North Korea, which has produced plutonium but lacks any real weapons capability, Pakistan is significantly ramping up its nuclear weapons programme.
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 were quoted as saying that the build-up is a "response to the threat from India, which is spending 50 billion dollars over the next five years on its military".
"But to say it's just an issue between just India and Pakistan is divorced from reality," said former senator Sam Nunn, who co-chairs the Nuclear Threat Initiative.
"The US and Soviet Union went through 40 years of the Cold War and came out every time from dangerous situations with lessons learned. Pakistan and India have gone through some dangerous times, and they have learned some lessons. But not all of them. Today, deterrence has fundamentally changed. The whole globe has a stake in this. It's extremely dangerous."