The US spy agency has quietly tightened its rules on drone strikes in Pakistan over concerns about their impact on tense relations with Islamabad, the Wall Street Journal reported Thursday.

The Journal, citing senior officials, said the new rules resulted from a behind-the-scenes battle between an aggressive Central Intelligence Agency and US military and diplomatic officials concerned about relations with Pakistan.

A high-level review reaffirmed support for the drone program -- which has killed hundreds of militants, including top commanders, in recent years -- but established new rules to minimize the diplomatic blowback, the Journal said.

The changes reportedly include granting the State Department greater sway in strike decisions, giving Pakistani leaders advance warning of more operations and suspending operations when Pakistani officials visit the United States.

"It's not like they took the car keys away from the CIA," the Journal quoted a senior official as saying. "There are just more people in the car."

The Journal said the debate was sparked by a particularly deadly drone strike on March 17 that took place just one day after Pakistan agreed to release a CIA contractor who had killed two Pakistanis.

Tensions between the two allies escalated throughout the spring, climaxing in May with the killing of Al-Qaeda chief Osama bin Laden in Pakistan in a secret US commando raid carried out without Islamabad's knowledge.

At issue in the debate over drones were so-called "signature strikes," in which unmanned drones fire on groups of suspected militants without necessarily knowing all their identities, and which make up the bulk of operations.

Such strikes are seen as more controversial than "personality" strikes, which target alleged top militants, the Journal said.

US officials do not publicly discuss the drone program, but claim to have substantially weakened Al-Qaeda in Afghanistan and Pakistan in recent months by taking out top leaders.

Pakistan has criticized the program, however, saying it inflames anti-American sentiment and extremism by killing scores of civilians.