US deployed its new stealth drone aircraft to fly dozens of high altitude secret missions deep into Pakistani airspace to monitor the Abbottabad compound where Osama bin Laden was killed, but was still unable to confirm his 100 per cent presence in the house.

"Using unmanned planes designed to evade radar detection and operate at high altitudes, the CIA conducted clandestine flights over the compound for months before the May 2 assault in an effort to capture high-resolution video that satellites could not provide," the Washington Post said.

The use of the stealth drones allowed the CIA to glide undetected beyond the boundaries that Pakistan has long imposed on other US drones, including the Predators and Reapers that routinely carry out strikes against militants near the border with Afghanistan, the daily said quoting current and former US officials.

The unmanned aircraft believed to be Lockheed Martin's new RQ-170 Sentinels were also deployed on the night of the raid providing imagery that President Barack Obama and members of his National Security team appear in photographs to be watching, as US Navy SEALs descended on the compound shortly after 1 a.m in Pakistan.

The drones were also used to eavesdrop on electronic transmissions, enabling US Commanders to monitor Pakistan's response. The Post, quoting Pentagon experts, said the new drones represents major advance in the capabilities of remotely piloted planes, which have been the signature American weapon against terror groups since the 9/11 attacks.

The existence of the advanced UAV programme was acknowledged by the US airforce in 2009, two years after it was spotted at an airbase in Kandahar, Afghanistan.

The new drones bear the distinct bat-winged shape of larger stealth warplanes and typically use a range of radar-defeating technology.

Their undersides are covered with materials designed to absorb sound waves rather than bouncing them back at sensors on the ground. Their engines are shielded and their exhaust diverted upwards to avoid heat trails visible to infrared sensors.

CIA turned to the new stealth aircraft 'because they needed to see more about what was going on' than other surveillance platforms allowed, a former US official familiar with the details of the operation was quoted as saying.

"It's not like you can just park a Predator overhead — the Pakistanis would know," added the former official, who, like others interviewed, spoke on the condition of anonymity, citing the sensitivity of the program.

"The CIA's repeated secret incursions into Pakistan’s airspace underscore the level of distrust between the United States and a country often described as a key counterterrorism ally, and one that has received billions of dollars in US aid," The Washington Post reported.

Despite all this the CIA never obtained a photograph of bin Laden at the compound or other direct confirmation of his presence before the assault. But it concluded after months of watching the complex that the figure frequently seen pacing back and forth was probably the al-Qaeda chief, the daily said.