Over the past decade, the US assessment of nuclear weapons security in Pakistan appears to have changed considerably from confidence to concern, particularly as a result of the introduction of tactical nuclear weapons, the report said.
"With several delivery systems in development, four plutonium production reactors, and its uranium enrichment facilities expanding, however, Pakistan has a stockpile that will likely increase further over the next 10 years," says the report.
Pakistan continues to expand its nuclear arsenal with more warheads, more delivery systems and a growing fissile materials production industry, it said.
"Analysis of a large number of commercial satellite images of Pakistani army garrisons and air force bases shows what appear to be mobile launchers and underground facilities that might be related to nuclear forces," said the report.
The authors observe that the size of the increase will depend on many factors.
Two key factors will be how many nuclear-capable launchers Pakistan plans to deploy, and how much the Indian nuclear arsenal grows.
"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 the growth rate over the past two decades," the authors said.
According to the report, Pakistan is modifying its nuclear posture with new short-range nuclear-capable weapon systems to counter military threats below the strategic level.
"The efforts seek to create a full-spectrum deterrent that is designed not only to respond to nuclear attacks, but also to counter an Indian conventional incursion onto Pakistani territory," it said.
"This development has created considerable concern in other countries, including the United States, which fears that it lowers the threshold for nuclear use in a military conflict with India," the report added.