WASHINGTON — Pakistan has begun building what independent analysts say is a powerful new reactor for producing plutonium, a move that, if verified, would signal a major expansion of the country’s nuclear weapons capabilities and a potential new escalation in the region’s arms race.

Satellite photos of Pakistan’s Khushab nuclear site show what appears to be a partially completed heavy-water reactor capable of producing enough plutonium for 40 to 50 nuclear weapons a year, a 20-fold increase from Pakistan’s current capabilities, according to a technical assessment by Washington-based nuclear experts.

The construction site is adjacent to Pakistan’s only plutonium production reactor, a modest, 50-megawatt unit that began operating in 1998. By contrast, the dimensions of the new reactor suggest a capacity of 1,000 megawatts or more, according to analysis by the Institute for Science and International Security. Pakistan is believed to have 30 to 50 uranium warheads, which tend to be heavier and more difficult than plutonium warheads to mount on missiles.

“South Asia may be heading for a nuclear arms race that could lead to arsenals growing into the hundreds of nuclear weapons, or at minimum, vastly expanded stockpiles of military fissile material,” the institute’s David Albright and Paul Brannan concluded in the technical assessment, a copy of which was provided to the Washington Post.