Pakistan has refused to comment on news that it is building a powerful new plutonium nuclear reactor, which could signal a major overhaul of the country's nuclear weapons program. A Washington-based research group released this week what it says are satellite photographs of what appears to be a new reactor under construction at a Pakistan nuclear site. 

The photographs, released on-line (on an Internet web site) Sunday evening, appear to show a large, partially completed nuclear reactor inside the Khushab complex in central Pakistan.

According to the Washington-based Institute for Science and International Security (ISIS), the heavy water reactor could potentially produce more than 200 kilograms of weapons-grade plutonium per year, enough for 40 to 50 nuclear weapons.

Speaking to reporters Monday, Pakistan's foreign ministry spokeswoman, Tasneem Aslam, would neither confirm nor deny the institute's findings, but downplayed the report's significance.

"I will not comment on the specifics, but Pakistan is a nuclear weapons state, it's a known fact. It is also a known fact that Khushab is hosting nuclear facilities," she said.

The report says construction of the new reactor likely began in early 2000, and could be completed in a few years.

The institute's assessment also suggests construction has been delayed by possible shortages in key materials, and says Pakistan is apparently not rushing to finish the project.

The news has raised concern about Pakistan's nuclear program and the possibility a new reactor could trigger an arms race with regional rival, India. Both nations maintain limited nuclear arsenals, and neither has signed the nuclear Non-Proliferation Treaty.

Pakistan Foreign Ministry spokeswoman Tasneem Aslam dismissed concerns of an arms race, however, stressing Islamabad's commitment to easing cross-border tensions.

"We have repeatedly said we do not want a nuclear or conventional arms race in the region and that remains our position: we do not want a nuclear arms race," added Aslam.

Aslam said the location of the Kushab nuclear site has already been given to India, under a long-standing agreement with New Delhi meant to reduce the risk of nuclear conflict.

The ISIS report also suggested India is likely aware of the new reactor's construction.

The U.S. Congress is about to debate a Bush administration proposal to share sensitive nuclear technology with India.

India has promised to use that technology only for peaceful purposes, and has agreed to strengthen internal safeguards on its civilian nuclear reactors.

Pakistan, which was not offered a similar deal by the Bush aministration, has sharply criticized the proposal, saying it could destabilize the region's delicate nuclear balance.

Pakistan's nuclear program has been mostly isolated internationally since 2004, when the country's top nuclear scientist admitted he illegally sold advanced weapons technology to a number of countries, including Iran and North Korea.