Islamabad: To ramp up the production of plutonium, Pakistan is expanding its nuclear weapons programme at a rapid pace and is expected to have a fourth operational reactor soon.

According to new commercial satellite imagery obtained exclusively by a news 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 by 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, however, refused to comment but a senior US congressional official working on nuclear issues told the magazine 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. And that nobody in the US or in the Pakistani government says anything about this – especially in this day and age – is perplexing, “said Paul Brannan of the Institute for Science and International Security.

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.

"There's no transparency in how the fissile material is handled or transported," said Mansoor Ijaz, who has played a role in back-channel diplomacy between Islamabad and New Delhi.

An unnamed Defence Department official said the US government is "confident that Pakistan has taken appropriate steps toward securing its nuclear arsenal".

"Pakistan knows it can outstare the West,” said Pakistani nuclear physicist Pervez Hoodbhoy.

Pakistani leaders defend their weapons programme as a strategic necessity since they cannot match India's military spending.

"Regretfully, there are several destabilizing developments that have taken place in recent years," Khalid Banuri of Pakistan's Strategic Plans Division, which maintains the nuclear arsenal, wrote in response to the magazine’s questions.

Among Pakistan's concerns, Banuri pointed to India's military build-up and the US's civilian nuclear deal with it.

"Our program is an issue of extreme sensitivity for every man, woman, and child in Pakistan," said former President Pervez Musharraf, adding that the nukes are "well dispersed and protected in secure locations".

To guard its strategic assets, Pakistan employs two army divisions — about 18,000 troops — and, as Musharraf put it: "If you want to get into a firefight with the forces guarding our strategic assets, it will be a very sad day."