Washington, May 6 (IANS) The US Central Intelligence Agency (CIA) conducted extensive surveillance of Osama bin Laden's hideout in Pakistan for months from a safe house in Abbottabad where the Al Qaeda leader was killed in a US raid.

The secret CIA facility relied on Pakistani informants and other sources to help assemble a "pattern of life" portrait of the occupants and daily activities at the fortified compound where bin Laden was found, the Washington Post reported citing unnamed officials.

The on-the-ground surveillance work was part of an intelligence-gathering push mobilised after the discovery of the suspicious complex last August, the daily said.

It involved virtually every category of collection in the US arsenal, ranging from satellite imagery to eavesdropping efforts aimed at recording voices inside the compound. The effort was so extensive and costly that the CIA went to Congress in December to secure authority to reallocate tens of millions of dollars within assorted agency budgets to fund it, officials were cited as saying.

The Post said US officials declined to say how many case officers or informants used the facility, but they stressed that the effort required extraordinary caution because of the fear that bin Laden and those sheltering him might vanish again if spooked.

The CIA began to focus on the compound last summer after years of painstaking effort to penetrate a small network of couriers with ties to the Al Qaeda leader.

Once the most important of those couriers led them to the Abbottabad compound, the conspicuous nature of the complex sent up alarms that it might have been built for bin Laden himself.

"The place was three stories high and you could watch it from a variety of angles," the Post cited a former official as saying. Moving into the custom-made compound, "was his biggest mistake," he added.

Most of that surveillance capability remained in place until the execution of the raid by US Navy SEALs shortly after 1 a.m. in Pakistan.

The agency's safe house did not play a role in the raid and has since been shut down, in part because of concerns about the safety of CIA assets in the aftermath, but also because the agency's work was considered finished, the Post said.

"The CIA's job was to find and fix," said a US official cited by the daily, using Special Operations forces terminology for the identification and location of a high-value target.