WASHINGTON: US intelligence officials have twice handed Islamabad tips about insurgent bomb-making factories, only to find them abandoned before Pakistani troops arrived, The Washington Post reported Friday.

The vacated factories have led US officials to question whether the information had been mistakenly leaked in recent weeks or whether the insurgents had been directly warned by Pakistan's Inter-Services Intelligence (ISI) agency, according to the report.

Relations between the two longtime allies have been seriously strained since US commandos raided a compound just a mile from a prestigious military academy deep in Pakistan, killing al Qaeda leader Osama bin Laden, who had been living there undetected for years.

The United States has been trying to bolster its relationship with Pakistan since the May 2 raid, and the information sharing with Islamabad is part of that effort.

The Post said Pakistani officials were given surveillance video in mid-May that located two bomb-making plants in the remote tribal areas of North and South Waziristan.

But by the time Pakistani troops arrived on June 4, the sites had been vacated.

A senior Pakistani military official said the United States had shared information about weapons storage facilities as well, but these had also been found empty.

“There is a suspicion that perhaps there was a tip-off,” the official told the newspaper.

“It's being looked into by our people, and certainly anybody involved will be taken to task.” The ISI was formed in 1948 shortly after independence from British rule, and has played a key role in Pakistan, which has spent more than half its 64-years existence under military rule.

But the West has long harbored suspicions that Pakistan, and particularly elements within the ISI, have failed to cut all ties with militants.

Western analysts suspect the ISI is divided, with some elements increasingly seeing militants as a domestic threat after Taliban and al Qaeda-linked bombings killed more than 4,240 people in Pakistan in the past four years.