David Albright of the Institute for Science and International Security said Thursday commercial satellite images show construction beginning on a building near three plutonium reactors at Khushab outside Islamabad. "It matches so many characteristics of the others that we characterize it as a likely fourth reactor," he said.
U.S.-based experts say Pakistan and India are locked in a long-running nuclear arms race, with both sides building about 10 warheads a year. Albright estimates that Pakistan may already have built 100 nuclear weapons, taking the lead over its neighbor and rival.
Islamabad is a key ally of the United States in its war in Afghanistan, but the South Asian country faces political and economic uncertainty as well as a continuing threat from domestic terror groups.
The U.S. and Pakistan have repeatedly insisted that Pakistan's stockpile of nuclear weapons and material is secure. A spokesman for Pakistan's foreign office recently criticized "unnecessary alarmist reporting about Pakistan's nuclear program."
But several independent experts have cautioned that as the nation's arsenal grows, so does the risk of theft or diversion. "The more weapons there are to guard, the more people get involved and therefore the more such risks" arise, said Michael O'Hanlon, senior fellow in foreign policy at the Brookings Institution in Washington.
O'Hanlon said Pakistan's nuclear program also drains scarce resources from other important efforts, such as shoring up the country's weak economy and battling militants along the Pakistan-Afghan border. "A burdensome nuclear arsenal is a worrisome thing," he said.
Hans M. Kristensen, director of the nuclear information project at the Federation of American Scientists, said each side claims to be building just enough weapons to have a deterrent force, but neither shows any signs of slowing down.
The problem, Kristensen said, is that neither country has specified "how much it requires to be enough."
Peter Crail, a nuclear nonproliferation analyst at the Arms Control Association, sees little danger of a nuclear conflict between India and Pakistan. But, he said, if Pakistan's government were overthrown "the risks might shoot through the roof."