A member of a new Syrian force trained by the U.S. military was believed to have been killed in clashes last week with al Qaeda's Syria wing, in what would be the fledgling force's first battlefield casualty, U.S. officials said on Monday.

The U.S. officials, who spoke to Reuters on condition of anonymity to discuss the incident, said the Syrian rebel was believed to have been killed during fighting on Friday with suspected members of Nusra Front. One of the officials described the information as preliminary.

The Pentagon declined to comment, citing "operational security reasons."

Friday's attack triggered the first U.S. air strikes to support the Syrian force. At the time, the U.S. military said the fighters repelled the attack, without citing casualties among the U.S.-trained force.

Defending the U.S.-trained fighters could become a growing job for United States, which has been waging air strikes against Islamic State targets in Syria.

U.S. officials disclosed to Reuters on Sunday that the United States has decided to allow air strikes to help defend against any attack on the U.S.-trained Syrian rebels, even if the attackers come from forces loyal to Syrian President Bashar al-Assad.

U.S. President Barack Obama has long sought to avoid any direct U.S. military confrontation with Assad's forces, focusing instead on the battle against Islamic State.

The Pentagon, State Department and White House have so far declined to publicly detail the rules of engagement in Syria.

Still, the Obama administration appeared on Monday to play down the chances that Assad's forces would target the U.S.-backed rebels and noted that his military had not fired on U.S.-led coalition aircraft bombing Islamic State targets in Syria.

"So far, we have seen the Assad regime abide by the admonishment that we have offered to not interfere with our activities inside of Syria," said White House spokesman Josh Earnest.

The U.S. military launched its program in May to train up to 5,400 fighters a year in what was seen as a test of Obama's strategy of getting local partners to combat extremists and keep U.S. troops off the front lines.

The training program has been challenged from the start, with many candidates being declared ineligible and some even dropping out. Obama's requirement that they target militants from Islamic State has sidelined huge segments of the Syrian opposition, which is focusing instead on battling Syrian government forces.

Only around 60 have been deployed to the battlefield so far.

The past week has illustrated that, in Syria's messy civil war, Islamic State is only one of the threats to the U.S. recruits.

The suspected militants from Nusra Front attacked U.S.-trained fighters on Friday at a compound in Syria, which was also being used by members of a Western-aligned insurgent group, known as Division 30, officials said.

The U.S. officials who disclosed the death of the U.S.-trained Syrian fighter said Division 30 also suffered casualties.

The United States is also preparing to start launching airstrikes against Islamic State from bases across the border in Turkey, after securing a long-sought agreement with Ankara.

The Pentagon said on Monday that the U.S. military started flying its first unmanned, armed drone missions from Turkey over the weekend. No strikes have been carried out yet.