Pakistan has almost as many nuclear weapons as its neighbour and rival India, and fears are increasing that some of its arms may fall into the hands of terrorists.
The Stockholm International Peace Research Institute says in a report issued last month that the two nations are locked in a weapons arms race and that each has 110 warheads.
But it says Islamabad is in danger of "losing control of part of its nuclear arsenal" to terrorists.
Describing Pakistan's nuclear programme as the fastest in the world, the report says Islamabad and New Delhi continue to develop new ballistic and cruise missile systems able to carry nuclear weapons.
"They are also expanding their capacities to produce fissile material for military purposes," it says, claiming that Pakistan is now turning to producing lighter precision warheads or tactical nuclear weapons for limited battlefield use.
Peace research institute director Daniel Nord cautioned that Pakistan was in danger of "losing control of part of its nuclear arsenal" to non-state parties.
And he said that South Asia, where relations between India and Pakistan were perpetually tense, was the only place in the world where there was an ongoing nuclear arms race.
The two nations conducted tit-for-tat underground nuclear tests in May 1998, Pakistan carrying out six explosions in response to India's five.
India conducted its first atomic test in 1974, calling it a "peaceful nuclear explosion".
This challenged the hegemony of the five nuclear weapon states of Britain, China, France, Russia and the United States, and triggered Pakistan's nuclear programme.
Since independence 64 years ago, India and Pakistan have fought three wars and an 11-week border skirmish in 1999 that threatened to escalate into a nuclear exchange.
The nuclear threat grew again two years later after the attack on India's Parliament by Pakistan-based Islamic terror groups.
Both countries stationed battle-ready armies along their common frontier for more than 10 months.
Then in January this year, Pakistan announced its intent to expand its nuclear deterrence against India. This was in response to the United States facilitating New Delhi's entry into the 45-member Nuclear Suppliers Group.
Pakistan's permanent representative to the Conference on Disarmament in Geneva declared that the decision to increase the size of its nuclear arsenal was made last December by the National Command Authority, which controls the country's atomic weapons.
Zamir Akram said India's admission to the Nuclear Suppliers Group would "destabilise the security environment in South Asia, which would retard progress on non-proliferation, arms control and disarmament measures".
It would enable India to expand its nuclear co-operation agreements and enhance its nuclear weapon and delivery capability.
In response, Pakistan would be "forced " to "ensure the credibility of its strategic deterrence", he stated.
"The National Command Authority categorically reiterated that Pakistan will never accept discriminatory treatment and that it rejects any effort to undermine its strategic deterrence," Akram said.
India has claimed its nuclear deterrence is directed against China, but Pakistan's is focused on India, a substitute for existing conventional military disparity between the constantly squabbling neighbours.
The arsenals' location remains a closely guarded secret, and the warheads and delivery systems are separated.
Pakistan has acquired missile technology from North Korea, and has smaller yield strategic weapons deliverable by modified F16 combat aircraft.