Suggesting that India has a sizeable nuclear weapons effort, a US think tank estimated India's nuclear arsenal at around 75-125 weapons made from weapon-grade plutonium and perhaps some thermonuclear weapons.
"India has a substantial stock of nuclear weapons made from weapon-grade plutonium, and perhaps some thermonuclear weapons that rely on both weapon-grade plutonium and weapon-grade uranium," according to the Institute for Science and International Security (ISIS).
"An estimate of India's nuclear arsenal can be derived by considering its weapon-grade plutonium stock," it said. "The resulting estimate has a median of 138 nuclear weapons equivalent with a range of 110 to 175 weapons equivalent."
However, the actual number of nuclear weapons India built from its stocks of weapon-grade plutonium must be less, ISIS said.
"When accounting for the amount of plutonium in the weapons production pipelines and in reserves, it is reasonable to assume that only about 70 per cent of the estimated stock of weapon-grade uranium is in nuclear weapons," it said.
"Thus, the predicted number of weapons made from its weapon-grade plutonium at the end of 2014 is about 97 with a range of 77-123," ISIS said. "These values are rounded to 100 nuclear weapons with a range of 75-125 nuclear weapons."
The think tank also noted that India has one of the largest nuclear power programmes among developing nations.
Utilising plutonium produced in these power reactors and discharged in irradiated or spent fuel, India has developed a relatively large civil plutonium separation programme and an associated fast breeder reactor programme that is using that separated plutonium, the report said.
For its "sizeable nuclear weapons effort", ISIS said, India uses "separated plutonium produced primarily in a set of small, dedicated reactors and a smaller amount produced in nuclear power reactors."
"It has a growing gas centrifuge programme able to produce significant amounts of highly enriched uranium (HEU) mostly for naval reactor fuel and perhaps for nuclear weapons, including thermonuclear weapons," it said.
Despite many obstacles, India has managed over several decades to put in place a relatively large nuclear weapons production complex, the report said.
"Its current complex can produce plutonium and highly enriched uranium for nuclear weapons and nuclear powered submarines," ISIS said. "It has a sophisticated missile production complex that provides the delivery systems for its nuclear weapons."
"Indian nuclear weapons use weapon-grade plutonium," the report said. "The bulk of this plutonium for nuclear weapons has come from the Cirus and Dhruva heavy water reactors, both located at the Bhabba Atomic Research Centre (BARC) in Mumbai."
ISIS cited an unnamed senior US official as saying that "after the 1998 tests, India used its civil power reactors to 'surge' weapon-grade plutonium production for its nuclear weapons programme."
"India explained to US officials at that time that it needed to build up its weapons plutonium stock after the 1998 tests before it engaged in negotiations for a Fissile Materials Cut-off Treaty (FMCT), negotiations which have still not come to fruition," the report said.
It may have subsequently produced additional weapon-grade plutonium for nuclear weapons in its civil power reactors, it said
"Although generally India is not believed to use reactor-grade plutonium in nuclear weapons, Indian nuclear experts are reported to have evaluated this plutonium's use in nuclear weapons and India may have decided to create a reserve stock of reactor-grade plutonium for possible use in nuclear weapons," the report suggested.