A Pakistani man approached CIA officers in Islamabad last year, offering to give up secrets

of his country's closely guarded nuclear program. To prove he was a trustworthy source, he claimed to possess spent nuclear fuel rods.

But the CIA had its doubts. Before long, the suspicious officers had concluded that Pakistan's spy agency, the Inter-Services Intelligence, was tryingto run a double agent against them.

CIA officers alerted their Pakistani counterparts. Pakistan promised to look into the matter and, with neither side acknowledging the man was a double agent, the affair came to a polite, quiet end.

The incident, recounted by former US officials, underscores the schizophrenic relationship with one of America's most crucial counter terrorism allies. Publicly, officials credit Pakistani collaboration with helping kill and capture numerous al-Qaeda and Taliban leaders. Privately, that relationship is often marked by mistrust as the two countries wage an aggressive spy battle against each other.

The CIA has repeatedly tried to penetrate the ISI and learn more about Pakistan' s nuclear program. The ISI has mounted its own operations to gather intelligence on the CIA's counter terrorism activities in the tribal lands and figure out what the CIA knows about the nuclear program.

Bumping up against the ISI is a way of life for the CIA in Pakistan, the agency's command centre for recruiting spies in the country's lawless tribal regions. Officers there also coordinate Predator drone airstrikes, the CIA's most successful and lethal counter terrorism program. The armed, unmanned planes take off from a base inside Pakistani Baluchistan known as "Rhine."

"Pakistan would be exceptionally uncomfortable and even hostile to American efforts to muck about in their home turf," said Graham Fuller, an expert on Islamic fundamentalism who spent 25 years with the CIA, including a stint as Kabul station chief.