The Institute for Science and International Security has obtained satellite images showing that a row of cooling towers at Pakistan's secret Khushab-III reactor has been completed, The Daily Telegraph reported.
This suggests the plant could begin operation within months, allowing Pakistan to increase substantially its stockpiles of weapons-grade plutonium, it said.
Work at Khushab-III has forged ahead even as Pakistan struggles to cope with floods that have inflicted damage estimated at 27 billion pounds - and amid mounting concerns over the long-term security of the country's nuclear arsenal, the report said.
Pakistan argues that its nuclear programme is necessary to counter the superior conventional forces of India, the report said.
A report, published by the Bulletin of Atomic Scientists, estimated it had assembled 70-90 nuclear warheads to India's 60-80, and had produced, enough fissile material to make a further 90.
According to the report, the US is also disturbed by Chinese plans to build two new nuclear reactors in Pakistan, bypassing rules that bar sales of nuclear equipment to states that have not signed the Nuclear Non-Proliferation Treaty.