The US official said it was not clear whether the attackers knew what they were targeting - and added that they would need to do much more to take control of Pakistan's nuclear arsenal.

'We are of the view these incidents did occur but we are not sure you can extrapolate a downgrade of security surrounding the Pakistani nuclear arsenal,' he said.

But resurgent jihadists in Pakistan still have the US worried over the safety of the country's nuclear weapons.

'The challenge to Pakistan's nuclear weapons from Pakistani Taliban groups and from Al Qaeda constitutes a real and present danger,' writes Shaun Gregory, director of the Pakistan Security Research Unit at the University of Bradford in Britain, in the West Point report.

Gregory's paper, 'The Terrorist Threat to Pakistan's Nuclear Weapons', notes that Pakistan's nuclear infrastructure was developed to ward off external attacks from India - not internal strikes from groups like the Taliban.