Pakistan is building a new nuclear reactor that could produce enough plutonium for 40 to 50 nuclear weapons a year in what would be a major expansion of its nuclear programme and could prompt an intensified arms race in South Asia, a report said.
But US officials and congressional aides, who confirmed the Pakistani plan, said it was unlikely to derail a nuclear cooperation accord with India or the sale of US-made F-16 jets to Islamabad.
News of the planned new Pakistani facility was confirmed as as the US Congress faced targets for action this week on both an Indian cooperation accord and F-16s deal.
"We have been aware of these plans, and we discourage any use of that facility for military purposes such as weapons development," White House spokesman Tony Snow told reporters.
He said the administration "discourage(s) expansion and modernization of nuclear weapons programmes, both of India and Pakistan," nuclear rivals who refused to sign the nuclear Non-proliferation Treaty.
While US officials knew about the reactor project, congressional aides said Congress was largely unaware until a report in the Washington Post on Monday citing an analysis of satellite photos and other data by the Washington-based Institute for Science and International Security.
The analysis concluded Pakistan was building a second larger heavy water reactor at its Khushab complex that could produce enough plutonium for 40 to 50 nuclear weapons a year.
Construction apparently began sometime after March 2000. But the analysis said Pakistan did not appear to be hastening completion, possibly due to shortages of reactor components or weapons production infrastructure.
The administration preferred to keep the project quiet because public disclosure "probably will aggravate concerns in India" as well as on Capitol Hill, one US official said.
Congress this week faces a deadline for acting if it wants to block administration plans to sell Pakistan up to 36 F-16C/D Block 50/52 Falcon fighters built by Lockheed Martin Corp in a deal potentially worth up to $5 billion.
Some lawmakers are concerned about Pakistan's past nuclear proliferation record and fear the warplane technology could be leaked to China, Pakistan's close ally.
Congress could block the sale by enacting a resolution of disapproval in both houses within 30 days of the June 28 notification date, but such action is rare.
But to survive a presidential veto, the legislation would have to pass both houses with a two-thirds majority.
"The reality ...is that it's very difficult to pass a resolution of disapproval," said Rachel Stohl of the Center for Defense Information.
Added a congressional aide: "There should be no effect on the sale of F-16s (because of the new reactor). So far there seem to be no major obstacles to the sale.
The US House of Representatives on Wednesday is to take the first of two key votes on the Indo-US nuclear deal, which would permit sales of American nuclear fuel and reactors to New Delhi for the first time in 30 years. US officials and congressional aides expect the deal to be approved.