Pakistan allowed Chinese military engineers to photograph and take samples from the top-secret stealth helicopter that US special forces left behind when they killed Osama bin Laden, the Financial Times has learnt.

The action is the latest incident to underscore the increasingly complicated relationship and lack of trust between Islamabad and Washington following the raid.

"The US now has information that Pakistan, particularly the ISI, gave access to the Chinese military to the downed helicopter in Abbottabad," said one person in intelligence circles, referring to the Pakistani spy agency. The Chinese engineers were allowed to survey the wreckage and take photographs of it, as well as take samples of the special "stealth" skin that allowed the American team to enter Pakistan undetected by radar, he said.

President Barack Obama's national security council had been discussing this incident and trying to decide how to respond. A senior official said the situation “doesn't make us happy”, but that the administration had little recourse.

As Navy Seals raided Bin Laden's compound in the military city of Abbottabad, just outside Islamabad, in May, one of their modified Black Hawk helicopters crashed into the wall of the compound, rendering it inoperable.

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