Pakistan appears to be increasing its production of nuclear materials with the apparent construction of a fourth reactor at its Khushab nuclear site, according to a US-based think tank.

Releasing satellite images from January 15, the Institute for Science and International Security said in a report Wednesday that the pictures showed the early construction of a fourth military nuclear reactor set to be the same size as two of the other buildings.

"Pakistan is determined to produce considerably more plutonium for nuclear weapons," ISIS said in its report, noting that since the announcement of its first reactor at the Khushab site in 1998, the nuclear power began constructing a second reactor around 2000-2002, and began building a third in 2006.

Pakistan has reportedly doubled its nuclear arsenal over the past several years, increasing its stocks to more than 100 deployed weapons.

Only four years ago the country's arsenal was estimated at 30 to 60 weapons, but has since stepped-up its production of plutonium and highly enriched uranium.

"They have been expanding pretty rapidly," ISIS president David Albright noted late last month, the Washington Post reported, with Islamabad edging ahead of its nuclear-armed rival India, estimated to have somewhere between 60 to 100 weapons.