The report also expressed concern that Islamabad's "full spectrum deterrence" doctrine has increased risk of nuclear conflict between the two South Asian neighbours.
"Pakistan's nuclear arsenal probably consists of approximately 110-130 nuclear warheads, although it could have more. Islamabad is producing fissile material, adding to related production facilities, deploying additional nuclear weapons, and new types of delivery vehicles," Congressional Research Service (CRS) said in its latest report.
In its 28-page report, the CRS noted that Pakistan's nuclear arsenal is widely regarded as designed to dissuade India from taking military action against it, but Islamabad's expansion of its nuclear arsenal, development of new types of nuclear weapons and adoption of a doctrine called "full spectrum deterrence" have led some observers to express concern about an increased risk of nuclear conflict between Pakistan and India, which also continues to expand its nuclear arsenal.
CRS is the independent research wing of the US Congress, which prepares periodic reports by eminent experts on a wide range of issues so as to help lawmakers take informed decisions.
Reports of CRS are not considered as an official view of the US Congress.
"Pakistan has in recent years taken a number of steps to increase international confidence in the security of its nuclear arsenal," said the CRS report authored by Paul K Kerr and Mary Beth Nikitin.
Moreover, Pakistani and US officials argue that, since the 2004 revelations about a procurement network run by former Pakistani nuclear official A Q Khan, Islamabad has taken a number of steps to improve its nuclear security and to prevent further proliferation of nuclear-related technologies and materials, it said.
A number of important initiatives, such as strengthened export control laws, improved personnel security, and international nuclear security cooperation programmes, have improved Pakistan's nuclear security, the CRS said.
"However, instability in Pakistan has called the extent and durability of these reforms into question. Some observers fear radical takeover of the Pakistani government or diversion of material or technology by personnel within Pakistan's nuclear complex," the CRS said.
"While the US and Pakistani officials continue to express confidence in controls over Pakistan's nuclear weapons, continued instability in the country could impact these safeguards. Furthermore, continued Indian and Pakistani nuclear weapons development could jeopardise strategic stability between the two countries," it concluded.
According to CRS, Pakistan has asserted that continued exclusion of the country from the NSG "would adversely affect regional peace, security and stability," as well as "undermine the global non-proliferation regime."
According to the US law, the United States could apparently advocate for Pakistan's NSG membership without congressional approval.
Ambassador Olson testified on December 16, 2015, that the Obama administration is "not negotiating... a civil nuclear cooperation agreement with...Pakistan."
However, press reports indicate that the United States is considering supporting Islamabad's NSG membership in exchange for Pakistani actions to reduce perceived dangers associated with the country's nuclear weapons programme.
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