A Washington-based research institute says Pakistan is building a nuclear reactor that could produce enough plutonium for 40 to 50 nuclear weapons a year.

The Institute for Science and International Security reports that satellite photos show a possible construction site for a larger nuclear reactor near the small one in the Khushab district of Punjab province.

The report says such a reactor could produce over 200 kilograms of weapons-grade plutonium, good for 40 to 50 nuclear weapons a year.

According to some media reports, Pakistan is now capable of producing plutonium for just two warheads a year.

Pakistan's Foreign Ministry spokeswoman Tasnim Aslam declined to say whether a new reactor is being constructed, but she said the presence of a nuclear weapons program and facilities at Khushab are well known.

Some information for this report was provided by AP and Reuters.