WASHINGTON - U.S. officials say Pakistan has apparently tipped off militants at two more bomb-building factories in its tribal areas, giving the terror suspects time to flee, after U.S. intelligence shared the locations with the Pakistani government.

Those officials believe Pakistan's insistence on seeking local tribal elders' permission before raiding the areas may have most directly contributed to the militants' flight. U.S. officials have pushed for Pakistan to keep the location of such targets secret prior to the operations, but the Pakistanis say their troops cannot enter the lawless regions without giving the locals notice.

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All officials spoke on condition of anonymity to discuss matters of intelligence.

The latest incidents bring to a total of four bomb-making sites that the U.S. has shared with Pakistan only to have the terrorist suspects flee before the Pakistani military arrived much later. The report does not bode well for attempts by both sides to mend relations and rebuild trust after the U.S. raid on May 2 that killed Osama bin Laden in Abbottabad, a Pakistani army town only 35 miles (56.32 kilometers) from the capital Islamabad.

The Pakistanis believe the Americans violated their sovereignty by keeping them in the dark about the raid. American officials believe bin Laden's location proves some elements of the Pakistani army or intelligence service helped hide the al-Qaida mastermind, bolstering their argument that the raid had to be done solo.

The U.S. officials explained Saturday how they first offered the location of the third, and then the fourth site, in order to give Pakistan another chance to prove it could be trusted to go after the militants.

In the tradition of 'trust but verify,' the Americans carefully monitored the area with satellite and unmanned drones, to see what would happen, after sharing the information a third and fourth time, the officials said.

In each case, they watched the militants depart within 24 hours, taking any weapons or bomb-making materials with them, just as militants had done the first two times. Only then, did they watch the Pakistani military visit each site, when the terror suspects and their wares were long gone, the officials said.

Pakistan's army on Friday disputed reports that its security forces had tipped off insurgents at bomb-making factories after getting intelligence about the sites from the United States. The army called the assertions of collusion with militants "totally false and malicious."

Army officials further claimed they had successfully raided two more sites, after finding nothing at the first two, but a Pakistani official reached Friday offered no details of what they found there.

The official admitted that in each raid, however, the Pakistani security services notified the local elders who hold sway in the tribal regions. The official said they would investigate U.S. charges that the militants had been tipped off.

Two U.S. officials said they were asking the Pakistanis to withhold such sensitive information from the elders, and even their lower ranks, to prove they could be trusted to keep a secret, and go after U.S. enemies.

At least two of the sites were run by the Haqqani network, which is part of the Taliban, closely allied with al-Qaida, and blamed for some of the deadliest attacks against U.S. troops and civilians in neighboring Afghanistan. Pakistan has long resisted attacking the Haqqani network, saying the group has never attacked the state of Pakistan.

The intelligence sharing was intended as a precursor to building a new joint intelligence team of CIA officers together with Pakistani intelligence agents. But U.S. officials say Pakistan has failed to quickly approve the visas needed, despite agreeing to form the team in May.

U.S. officials have also accused Pakistan of holding up to five Pakistani nationals accused of helping the CIA spy on the Abbottabad compound in advance of the bin Laden raid.

While not confirming the number, a Pakistani official said any citizen who worked with the U.S. to spy on the compound had betrayed his or her country by failing to tip off the government that someone the Americans wanted was hiding in the compound. Such a tip, the official said, could have saved the Pakistani government the embarrassment of being surprised by the bin Laden raid.