"Some observers fear radical takeover of a government that possesses a nuclear bomb, or proliferation by radical sympathizers within Pakistan's nuclear complex in case of a breakdown of controls.
"While US and Pakistani officials continue to express confidence in controls over Pakistan's nuclear weapons, continued instability in the country could impact these safeguards," it said.
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, the CRS report says.
The Congressional report running into nearly 30 pages informs US lawmakers that 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.
"For example, Peter Lavoy has argued that India's efforts to improve its conventional military capabilities could enable New Delhi to achieve technical superiority in intelligence, surveillance, and reconnaissance, as well as precision targeting, providing India with the capability to effectively locate and efficiently destroy strategically important targets in Pakistan," it said.
"Islamabad could respond by lowering the threshold for using nuclear weapons, according to Lavoy.
"Indeed, a Pakistan Foreign Ministry spokesperson warned in May 2009 that Islamabad could take this step.
"The Pakistani government may also consider fielding non-strategic nuclear weapons in order to increase the credibility of its nuclear deterrent versus Indian conventional military operations," the CRS report said.
According to the report, Pakistan has two types of delivery vehicles for nuclear weapons: aircraft controlled by the Pakistan Air Force and surface-to-surface missiles controlled by the Pakistan Army.
"Pakistan could deliver its nuclear weapons using F-16 fighter aircraft purchased from the United States, provided that modifications are made. It is widely believed that Islamabad has made the relevant modifications to the F-16s previously sold to them by Washington, it added.
