The international coalition confronting Libyan leader Moammar Gaddafi agreed Thursday to put NATO in charge of enforcing a no-fly zone but was still working on a deal to relieve U.S. forces of command of all military operations in the country.

U.S. officials said Secretary of State Hillary Rodham Clinton had reached preliminary agreement on a broader accord in a phone call earlier Thursday with her counterparts from Turkey, France and Britain. But Turkey raised last-minute objections Thursday evening, a diplomat said.

The Obama administration is eager to transfer military command to NATO to portray the six-day-old operation as an international humanitarian mission, rather than a U.S.-led offensive in another Muslim country.

Obama is also facing mounting pressure from Congress about the operation’s goals and the extent of U.S. involvement. The president has promised to turn over control of the mission within “days, not weeks,” with the United States assuming a supporting role.

Whatever happens in NATO, the coalition may continue to have a U.S. face. NATO Secretary General Anders Fogh Rasmussen said Thursday that leadership of the operations in Libya would be determined by the alliance’s normal command structure, which would mean an American officer would be in charge. However, a senior U.S. official said Thursday night that the task force overseeing the Libya operation would be led by a Canadian, Lt. Gen. Charles Bouchard.

Rasmussen said that NATO had agreed only to assume responsibility for enforcing a no-fly zone and an arms embargo against Libya. Alliance members were debating whether NATO would take on the more controversial role of protecting civilians on the ground — by striking Libyan tanks and troops, not just its planes, he said.

“We have not decided yet whether we will take on the broader responsiblity,” Rasmussen said in an interview with CNN’s “The Situation Room.”

He played down the possibility of a split, saying that “there is unity within NATO.”

American diplomats said NATO members had agreed in principle on a broader deal to include the entire civilian protection mission. Clinton said Thursday evening that the 28 NATO countries had authorized military authorities to develop a plan for that operation.

A senior administration official said in a conference call with reporters that the military operations plan would be finished over the weekend. “Then we need to execute it. The key issue here is a political agreement. Up to this point, there was no agreement in the alliance,” he said, speaking under ground rules of anonymity.

One Western diplomat said, however, that the Turks had balked on reaching a final agreement because of their uneasiness with the coalition’s ground attacks, which have raised concerns in the Arab world over possible civilian deaths.

“The Turks don’t like the [air]strikes,” the diplomat said, speaking on the condition of anonymity.

As the diplomatic maneuvering intensified, a second Arab country announced that it would send fighter jets to help enforce the no-fly zone. The United Arab Emirates pledged 12 aircraft, joining Qatar in the mission. The Obama administration has been eager to enlist Middle Eastern states to counter perceptions that Western powers are attacking an Arab state.

U.S. and other coalition forces continued to pound Libyan ground forces mounting offensives against civilians and rebels in several cities east of Tripoli. In addition, a French warplane fired a missile at a Libyan military aircraft as it was landing in the disputed city of Misurata, blowing it up, according to the French defense ministry.

Allied planes have avoided striking targets inside the cities for fear of killing civilians, even though the current rules of engagement allow for such attacks.

Allied commanders are betting that sustained strikes on supply lines and communications capabilities outside urban areas will wear down Gaddafi’s troops. The airstrikes do not appear to have swung the balance of power in favor of opposition forces in Misurata and Ajdabiyah.

“It is a very, very hard task,” said Vice Adm. Bill Gortney, director of the Joint Staff in the Pentagon.

A spokesman for Libya’s opposition forces said that some pro-Gaddafi forces in Ajdabiyah had offered to surrender.

“We are trying to negotiate with these people in Ajdabiyah because we are almost sure that they have lost contact with their headquarters,” said Col. Ahmad Omar Bani, a former Libyan army air force pilot. An imam was helping in the talks, he said.

U.S. officials said they had not seen any defections by the Libyan forces.

American diplomats said that the coalition had agreed in principle to set up a structure in which NATO’s military command would oversee offensive operations while its political body, the North Atlantic Council, would provide overall guidance. Non-NATO countries would participate in the structures, much as they do as part of the coalition fighting in Afghanistan, diplomats said.

“Our demands have been met on Libya. The operation will be handed over to NATO,” said Turkish Foreign Minister Ahmet Davutoglu, speaking on Turkish state television.

Turkey is the only Muslim-majority member of NATO, which requires that all members approve any military action.

If NATO does assume full control over the Libyan operations, the United States would still play a prominent role in the mission, providing air refueling tankers and surveillance planes, officials said.

American aircraft would also continue to fly combat strike missions after the transfer, Gortney said.

Clinton hailed the early successes of the coalition in destroying Libyan air defenses and pushing back loyalist forces from Libya’s second-largest city.

“A massacre in Benghazi was prevented,” she said.

In Tripoli, officials took journalists to a hospital Thursday to see the charred and mangled bodies of 18 men they said were victims of Western airstrikes, Reuters news agency reported. A Libyan official said that some of the dead were soldiers killed in an airstrike the day before and that some were civilians. But it was not possible to determine whether any civilians were among the dead.

Faced with an increasingly dire situation in Misurata, the Paris-based group Doctors Without Borders secretly delivered a boatload of medical supplies to the city Monday night, an official with the group said. The city had no electricity or running water, and the hospital was struggling to cope with the wounded, residents reported.

Staff writer Tara Bahrampour in Benghazi, correspondent Liz Sly in Tripoli and staff writers Joby Warrick and Scott Wilson in Washington contributed to this report.