Pakistan currently has between 25 and 50 nuclear weapons, mostly relatively simple uranium arms with "modest" yields around the size of the bombs that destroyed Hiroshima and Nagasaki, a journal claimed Thursday.

The Nature magazine's claim followed media reports that satellite photos of Pakistan's Khushab nuclear site have shown what appears to be a partially completed heavy-water reactor capable of a 20-fold increase from its current nuclear capabilities.

Quoting John Pike, director of globalsecurity.org, a non-profit group that specialises in image analysis, Nature says if the new facility is what it seems to be, it would allow Pakistan to build a lot more bombs.

The reactor is "gigantic" and would allow Pakistan to increase its total number of weapons tenfold, he says. Plutonium can be used to construct smaller and more lightweight weapons than uranium. Most uranium bombs require 15 to 20 kilograms of material, but plutonium weapons can be built with as little as 5 kilograms. That makes it easier to fit plutonium warheads on missiles.

In addition, small plutonium bombs are often used to trigger larger hydrogen weapons. So the technology, says Pike, is an important step towards developing those bombs, which are thousands of times more powerful than uranium and plutonium weapons.