Bulletin of Atomic Scientists in its latest report has said that Pakistan is expanding its nuclear arsenal and has developed an estimated stockpile of 130 to 140 warheads for delivery as well as converting some of its fighter jets, including F-16s to deliver nukes.
Authored by Hans M Kristensen and Robert S Norris, the report further said that analysis of a large number of commercial satellite images of Pakistan army's garrisons and air force bases shows what appear to be mobile launchers and underground facilities that might be related to nuclear forces.
"Pakistan continues to expand its nuclear arsenal with more warheads, more delivery systems, and a growing fissile materials production industry," said the report on Pakistani nuclear forces, 2016.
"We estimate that Pakistan now has a nuclear weapons stockpile of 130-140 warheads. This stockpile exceeds the projection made by the US Defense Intelligence Agency in 1999 that Pakistan by 2020 would have 60-80 warheads," said the report released last month.
According to the scientists, with several delivery systems in development, four plutonium production reactors and its uranium enrichment facilities expanding, Pakistan's stockpile will possibly increase further over the next 10 years.
"Speculation that Pakistan may become the world's third-largest nuclear weapon state - with a stockpile of some 350 warheads a decade from now - are, we believe, exaggerated, not least because that would require a buildup two to three times faster than growth over the past two decades," it said.
"We estimate that its stockpile could more realistically grow to 220-250 warheads by 2025, if the current trend continues. If that happens, it would make Pakistan the world's fifth-largest nuclear weapon state.
"But unless India significantly expands its arsenal or further builds up its conventional forces, it seems reasonable to expect that Pakistan's nuclear arsenal will not continue to grow indefinitely but might begin to level off as its current weapons programmes are completed," the report said.
According to the report, Pakistan probably assigns a nuclear strike mission to select F-16A/B and Mirage III/V fighter squadrons.
The F-16 was probably the first aircraft in the nuclear role, but the Mirage quickly joined the mission, it said, adding that the F-16A/Bs were supplied by the US between 1983 and 1987.
After 40 aircraft had been delivered, the US State Department told Congress in 1989, "none of the F-16s Pakistan already owns or is about to purchase is configured for nuclear delivery" and Pakistan "will be obligated by contract not to modify" additional F-16s "without the approval of the United States," it said.
"Yet, there were multiple credible reports at the time that Pakistan was already modifying US-supplied F-16s for nuclear weapons," it said, adding that there are rumors that Pakistan intends to make the Chinese-supplied JF-17 fighter nuclear-capable.
(With PTI inputs)