The U.S. military is expanding its combat role in Syria in defense of coalition-backed rebels, conducting armed drone missions into Syria from an air base in Turkey and imposing new rules allowing the U.S. to defend rebels against attacks from any hostile force, including the Assad government.

The first armed drone missions out of Turkey began last weekend, and the military is planning to add manned aircraft flights from there, U.S. officials said.

So far none of the drones launched airstrikes, but Navy Capt. Jeff Davis, a Pentagon spokesman, said they may begin conducting strikes soon. And he said the U.S. also will likely fly search and rescue missions from Incirlik Air Base in Turkey.

The use of the Incirlik base comes as the U.S. began launching airstrikes to defend U.S.-backed Syrian rebels when they come under fire from forces other than Islamic State militants. Davis said that on Friday, for the first time, the U.S. used strikes to defend rebels that were coming under attack by the al-Nusra Front, al-Qaida's branch in Syria.

Under current guidelines, U.S. forces can only conduct offensive missions against the Islamic State, and can't go after other groups such as Assad government forces, largely due to worries it would set off war with Syria.

But U.S. officials in recent weeks have walked a careful line, arguing that they have an obligation to defend the Syrian rebels that have been trained by the U.S. to help battle the Islamic State. So, under new rules, the U.S. can defend the rebels if they come under fire from government forces or any other group, such as the al-Nusra Front. But American forces still can't launch offensive missions against those other groups.

"I am pleased that the administration has finally decided to provide additional military support to the Syrian forces the Department of Defense is training and equipping to counter ISIL in northern Syria," said Sen. John McCain, using an alternate acronym for the extremist group that has overtaken parts of Syria and Iraq. But the Arizona Republican expressed concern about "a policy of gradual escalation that has plagued U.S. efforts since the start of the conflict in Syria."

Davis declined to provide details on the new rules of engagement for the military, including what conditions have to exist before the U.S. can defend the Syrian rebels, and whether strikes would be limited to instances when only U.S.-trained rebels are at risk, or if other rebels who have not yet been trained can also be defended.

The shift in military strategy had been pushed by members of Congress who argued that the U.S. couldn't train the rebels then send them into battle against the Islamic State with no back-up. Others, however, warned that many of the rebel groups are primarily focused on overturning the Syrian government, and that supporting them would put the U.S. into a messy war with a sovereign nation that has sophisticated air defenses.

Syrian President Bashar Assad last month acknowledged that his troops have lost territory to rebel forces and were running short on manpower, but he vowed that he would win the long-running civil war.

Until now, the U.S.-backed campaign against the Islamic State has aligned with Assad's interests, because he also wants to defeat the insurgents. So, his government forces have not interfered with any of the coalition aircraft launching strikes against IS. But the plan to now back the moderate rebels if they come under fire by regime forces could increase that risk and fuel opposition from other countries.

Russian Foreign Minister Sergey Lavrov, speaking to reporters after a meeting with Secretary of State John Kerry and Saudi Foreign Minister Adel al-Jubeir, criticized U.S.-led efforts to train members of the Syrian opposition to fight inside the country, and called for an end to "foreign intervention" in the Syria crisis.

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Associate Press writers Adam Schreck in Doha, Qatar, and Sagar Meghani in Washington, D.C., contributed to this report.