Sleuthing with satellites images on Google Earth has revealed a substantial and undocumented expansion to a suspected nuclear processing plant in Pakistan, in what researchers say is a possible sign of the country boosting capacity for its nuclear weapons programme.

Pakistan has possessed nuclear weapons since 1998 but is not a signatory to key international nuclear treaties on proliferation and tests. The country’s secretive nuclear weapons programme is closely watched due to tensions with neighbouring India, which also has a nuclear arsenal. …