"Pakistan appears to be increasing its fissile production capability and improving its delivery vehicles in order to hedge against possible increases in India's nuclear arsenal.
"Islamabad may also accelerate its current nuclear weapons efforts," the independent Congressional Research Service (CRS) said in a report.
In addition to making qualitative and quantitative improvements to its nuclear arsenal, Pakistan could increase the number of circumstances under which it would be willing to use nuclear weapons, the report said.
In its latest report, prepared for the US lawmakers, CRS said that Pakistan's nuclear weapons program is mainly aimed to address the threat perception it has from India and thus act as a deterrent from India.
"India has stated that it needs only a "credible minimum deterrent", but has never defined what it means by such a deterrent and has refused to sign the Comprehensive Test Ban Treaty," it said, adding that Pakistani officials have stated that the government may need to increase significantly its nuclear arsenal in response to possible Indian plans to do the same.
Besides lowering the threshold for using nuclear weapons, Pakistan may also consider fielding non-strategic nuclear weapons in order to increase the credibility of its nuclear deterrent versus Indian conventional military operations.
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Dated July 23, the CRS in its report said Pakistan's nuclear arsenal probably consists of approximately 90-110 nuclear warheads, although it could be larger.
"Islamabad is producing fissile material, adding to related production facilities, and deploying additional delivery vehicles," it said, adding these steps could enable Pakistan to undertake both quantitative and qualitative improvements to its nuclear arsenal.
"Whether and to what extent Pakistan's current expansion of its nuclear weapons-related facilities is a response to the 2008 US-India nuclear cooperation agreement is unclear.
"Islamabad does not have a public, detailed nuclear doctrine, but its 'minimum credible deterrent' is widely regarded as designed to dissuade India from taking military action against Pakistan," CRS said