The visits came after Libyan officials contacted US and British officials in March, initiating nine months of secret talks that culminated Friday with Libya's pledge to renounce weapons of mass destruction, the sources said.
US and British officials saw chemical weapons and signs of a relatively advanced nuclear program in more than ten site visits in October and December, according to a senior White House official who briefed reporters on condition of anonymity.
A CIA report to Congress released in November said that Libya had made its greatest strides with chemical weapons but had made little progress on its longstanding goal of acquiring or developing a nuclear weapon. The site visits contradicted the latter assessment, however.
"On the nuclear side, my understanding is that they did have a much further advanced program, including centrifuges," devices used to enrich uranium for use in nuclear weapons, the official said.
Building a centrifuge "is the long pole in the tent. Once you can do that, you can build a bomb," another administration official said, also speaking privately. "They told us that they had this program that was ultimately intended to produce a bomb."
Libya was trying to put together a team of foreign experts to build a nuclear weapon, the second official said, declining to say what countries where being courted.
Libyan officials also showed their visitors "a significant quantity" of mustard gas and aerial bombs that could be filled with mustard gas, as well as a dual-use capacity to produce mustard gas and nerve agents.
In nearly-joint surprise announcements late Friday, US and British leaders said Libyan leader Colonel Moamer Kadhafi had agreed to renounce all weapons of mass destruction and "immediately and unconditionally" welcome international inspectors.
The announcement coincided with Libya's acknowledgement that it had sought to develop WMD capabilities, as well as longer range missile, British Prime Minister Tony Blair said.