The American ambassador to Islamabad phoned Washington with an urgent plea: Stop an imminent CIA drone strike against militants on the Pakistani side of the Afghan border.

He feared the timing of the attack would further damage ties with Islamabad, coming only a day after the government grudgingly freed a CIA contractor held for weeks for killing two Pakistanis.

Ambassador Cameron Munter's rare request — disclosed to The Associated Press by several U.S. officials — was forwarded to the head of the CIA, who dismissed it.

Some U.S. officials said Leon Panetta's decision was driven by a belief that the militants being targeted were too important to pass up, but others suspected that anger at Pakistan for imprisoning Raymond Davis for so long played a role.

The deadly March 17 attack, which the Pakistanis claim killed 38 civilians, helped send the U.S.-Pakistan relationship into a tailspin from which it has not recovered.

The timing of the strike — and others that followed — outraged Pakistani officials, complicating U.S. efforts to win Pakistani cooperation on the Afghan war and retain support for the drone program.

Newly revealed details of the drone strikes were provided by U.S. and Pakistani officials who spoke on condition of anonymity because of the sensitivity of the program.

Among them were attacks that followed an April visit by Pakistan's spy chief to Washington as well as trips here by Sen. John Kerry and Secretary of State Hillary Rodham Clinton after the American raid that killed Osama bin Laden in a Pakistani military town in May.

The latest strike occurred Tuesday while the U.S. special envoy to Pakistan and Afghanistan, Marc Grossman, was visiting Islamabad.

Seven years into a secret program that has killed scores of al-Qaida and Taliban fighters, there are increasing questions over whether it is worth the diplomatic backlash in Pakistan.

President Barack Obama has dramatically ramped up the program, unleashing more than 200 strikes since he took office compared to fewer than 50 during the Bush administration.

The Pakistani government is widely believed to have supported the program in the past and even allowed the drones to take off from bases inside Pakistan, but that support has waned as relations between the two countries have soured.

The attacks have also strained the relationship between the U.S. State Department and the CIA, where officials argue that killing militants who threaten U.S. interests should take priority over political considerations, said U.S. officials.

That tension was clearly visible between Ambassador Munter and the CIA station chief in Islamabad, who recently left his post because of illness, said a senior Western official in the region.

"When the doors are closed they are shouting at each other, but once the doors are open they are congenial in front of the embassy staff," said the official.

The hard-charging station chief also clashed with the head of Pakistan's main intelligence agency, the ISI, over drone strikes, said a Pakistani official.

The CIA does not comment on the drone program. A U.S. official familiar with the issue played down the tension.

"It is very, very rare for the chief of mission to express concern about any particular operation," the official said, referring to the ambassador. "When concerns are raised, they're always given close consideration."

Munter must sign off on every planned drone attack in Pakistan, although he rarely voices an objection, said a former aide to the ambassador.

If Munter disagrees with a planned strike, the CIA director can appeal to him, said two U.S. officials, providing the most detailed description of the process to date.

Clinton can also weigh in, and has done so at least once, one U.S. official said.

On March 17, Munter used the embassy's secure line in an attempt to stop an imminent drone strike. His concern was that the strike — a day after the release of the CIA contractor Davis — would set back Washington's already shaky relations with Islamabad, said the former aide and a senior U.S. official.

The Davis case had left bad feelings on both sides. On Jan. 27 in Lahore, Davis shot to death two Pakistanis who he said were trying to rob him, enraging many people in a country where anti-American sentiment is high.

The U.S. insisted Davis had immunity from prosecution, but he was not released until March 16 under a deal that compensated the victims' families.

Pakistan's security agencies came under intense domestic criticism for freeing him.

Munter's request went to the State Department and was forwarded to then-CIA director Panetta, now secretary of defense, who insisted on going ahead, said the officials. It is unclear whether Clinton was involved in the decision.

The former aide said the strike reflected the CIA's anger at the ISI, which it blamed for keeping Davis in prison for seven weeks.

"It was in retaliation for Davis," the aide said. "The CIA was angry."

A U.S. official said all decisions on strikes are driven only by intelligence. That official spoke on condition of anonymity to discuss matters of intelligence.

The CIA also believed it was vital to kill the militants targeted in the strike in the Datta Khel area of North Waziristan, said the senior U.S. official. But other U.S. officials agreed with Munter that it wasn't worth the political blowback, the official said.

At about 11 a.m., two pairs of missiles were fired three minutes apart, hitting several dozen tribesmen meeting in the open in Shiga village near the Afghan border.

Pakistani officials and local tribesmen said four Taliban fighters and 38 innocent people were killed.

The CIA claimed they were all militants, but villagers and Pakistani officials said the group was holding a community meeting, or jirga, to resolve a local mining dispute.

A tribal elder, Malik Dawood, had purchased rights to cut down and sell a large tract of oak trees, said 40-year-old farmer Gul Ahmed.

But he subsequently realized the land contained chromite and argued with the landowner about whether he could mine it, he said.

Four Pakistani Taliban militants were attending the jirga to guarantee any decision made because of their control over the area, said the villagers and Pakistani officials.

U.S. officials said the CIA tracked the militants driving to the meeting and decided rather than targeting just the car, they would wait to get the entire assembled party.

The strike killed the militants, along with six tribal policemen and 32 other tribesmen, according to Ahmed, who provided the names of the dead and attended their mass funeral. A senior official in the area confirmed the death toll.

In a rare public statement, Pakistan's powerful army chief, Gen. Ashfaq Parvez Kayani, said the jirga "was carelessly and callously targeted with complete disregard to human life."

U.S. intelligence officials brusquely dismissed the Pakistani claims.

"There's every indication that this was a group of terrorists, not a charity car wash in the Pakistani hinterlands," said one official at the time.

ISI chief Lt. Gen. Ahmed Shuja Pasha took the strike as a personal insult because he had stepped in to get Davis released, Pakistani officials said.

The strike hampered counterterrorism cooperation between the CIA and the ISI, and the Pakistani government started sending U.S. military trainers home — a process that accelerated after the raid that killed bin Laden.

Pasha made a personal trip to Washington in April in an attempt to repair relations.

The ISI chief said he would work to let in more CIA operatives if the U.S. would consider including Pakistan in the process of drone strike targeting, said U.S. officials at the time.

But before Pasha had returned home, two U.S. missile strikes killed six suspected Taliban fighters in the South Waziristan tribal area.

Pakistani officials said the attacks were seen as another slap to Pasha and made it impossible for him to raise the CIA's requests with the army or the government.

This pattern continued after the U.S. raid that killed bin Laden in the Pakistani town of Abbottabad on May 2. The operation outraged the Pakistani government because it was not told about it beforehand.

Kerry, chairman of the Senate Foreign Relations Committee, visited in mid-May trying to salvage the relationship.

He made some progress but just as he left on May 16, the CIA launched a missile strike in North Waziristan, killing seven suspected militants.

In response, Pakistani army chief Kayani and President Asif Ali Zardari sent Kerry angry messages that he received when he touched down in Dubai, said Kerry's spokeswoman Jennifer Berlin, confirming details that first appeared in The New York Times Magazine.

State Department officials were also angry about three missile strikes that followed Clinton's visit to Pakistan at the end of May, said a U.S. official familiar with the events.

The prevailing view at the State Department and the White House is that CIA strikes are motivated by a drive to kill as many militants as possible in what the U.S. sees as a window of opportunity that might soon close, rather than a deliberate attempt to torpedo diplomacy, said the official.

White House adviser on Pakistan and Afghanistan Douglas Lute suggested as much during remarks last weekend at the Aspen Security Forum when he said al-Qaida was on its heels after the death of bin Laden.

Referring to the drone campaign only obliquely, as a "covert action program," Lute said, "I would not adjust programs today that are designed to go for the knockout punch when we've got this opportunity."

However, former U.S. intelligence chief Dennis Blair said the U.S. should stop its drone campaign in Pakistan because the strikes damage the U.S.-Pakistan relationship and are more of a nuisance than a real threat to al-Qaida.

"I just see us with that strategy walking out on a thinner and thinner ledge," Blair said at the forum, "and if even we get to the far end of it, we are not going to lower the fundamental threat to the U.S. any lower than we have it now."