The facility is in Karachi and was built through the US Atoms for Peace programme under President Dwight Eisenhower.

Mr Basit told the BBC: "The US has been asking for the last two years that we should return the reactor and the fuel which had been given.

"We dismissed this, reiterating that the reactor was our property, so there was no question of it being returned."

US officials have long expressed concern that Islamist militants in Pakistan could target the country's nuclear programme in an attempt to get their hands on the materials to build their own bomb.