Pakistan says it has conducted five nuclear tests since 1998 and is believed to have enough enriched uranium to produce between 50 and 110 warheads.
It has aircraft and missiles capable of delivering nuclear weapons, although some experts believe the missiles are not fully assembled, with the warhead stored away from the main body.
In recent years Pakistan has sought to acquire so-called dual use technology, which could be used for either civilian or military nuclear purposes.
Pakistan's difficult relationship with neighbouring India, and the apparent aim of both countries to develop nuclear arsenals, has periodically alarmed the international community.
The revelation in 2004 that its former chief nuclear scientist AQ Khan had led an international network which covertly shared nuclear technology among countries such as Iran, Libya and North Korea also raised grave concerns.