Recent satellite images obtained by Fox News from the aerial imagery company GEOEYE show an expanding nuclear facility at the Khushab facility about 100 miles south of Pakistan's capital city of Islamabad. The pictures were taken on April 20 and indicate Pakistan is building a fourth nuclear reactor to produce plutonium.

Paul Brannan, a nuclear analyst with the Institute for Science and International Security, explains how U.S. officials and other nuclear experts have been able to document the alarming expansion of Pakistan’s nuclear program in the past two years.

“They have a telltale sign,” Brannan said, looking at the images. “I mean you can see the square of the reactor building, you can see the inner square of the reactor hall where the actual reactor goes, and if you measure the dimensions of the building it matches up exactly to the second and the third reactors.”

In 2009 it was a barren site.

Pakistan first revealed the Khushab site and its plutonium production facility in 1998 after the country’s first nuclear test. That is when Pakistan’s official nuclear program turned from the highly enriched uranium model established by the father of that program, A.Q. Khan, to plutonium production. Plutonium weapons are lighter, more mobile and easier to deliver than those based on highly enriched uranium.

Pakistan already has 100 nuclear weapons. Experts, such as Brannan, say that the new reactors could give Pakistan enough fissile material to build somewhere between eight and 20 more nuclear weapons per year.

But it’s the cost of the reactors and the speed at which they have been built in the last two years that is raising eyebrows. Just how high is the cost? “It would be in the billions,” Brannan said. “This is a military reactor. It's outside of the civilian program.”

U.S. officials have been watching the Khushab facility for some time, but say there is no good explanation how Pakistan is paying for this. The United States has given Pakistan $20 billion in military and economic aid since September 11, 2001. Sen. John Kerry, who chairs the Foreign Relations Committee, returned from Islamabad Tuesday morning after Pakistan agreed to return the stealth helicopter tail of the downed Blackhawk MH-60 damaged in the raid that killed Usama bin Laden.

On Tuesday, Kerry warned against cutting off aid to Pakistan.

“Right now, we have about 100,000 reasons for worrying about our relationship with Pakistan,” Kerry said during a hearing with former National Security Adviser Gen. Jim Jones. “And they're called our young men and women, and they're in uniform in Afghanistan.”

Pakistan controls 70 percent of the supply routes into Afghanistan, according to Pentagon officials. Defense Secretary Robert Gates and others have warned that cutting off relations or aid to Pakistan would be dangerous because the government and military of Pakistan could fall under more influence from Islamic radicals who are sympathetic to Al Qaeda and those proliferating nuclear weapons could fall into the wrong hands.