WASHINGTON: Syria’s Al-Qaeda affiliate Nusra Front abducted five more rebels believed to have been trained by the United States after raiding areas along the Turkish border in the past 24 hours, a monitoring group said Tuesday.

The British-based Syrian Observatory for Human Rights said the militant group was now holding a total of 13 men from “Division 30,” a Syrian rebel unit that includes some fighters recently trained by the U.S. military.

It was not immediately clear whether the five men abducted overnight had been on the new U.S. military training programme or on an existing, separate CIA-led program.

On Friday, Nusra Front said in a statement it had detained a number of Syrian rebels trained by the United States and warned any others they should abandon the program.

The Pentagon said at the time that no members of an initial group of around 60 U.S.-trained rebels had been abducted.

However, a U.S. official, speaking on condition of anonymity, said Tuesday that the United States believes that at least five Syrian rebels who were trained by the U.S. military have been captured by the Nusra Front.Details surrounding the capture of the U.S.-trained fighters were still unclear, including the precise timing.

Two U.S. defense officials said it took place in the days after Friday’s attack by Nusra against some of the U.S.-trained Syrian rebels and the Western-aligned Syrian opposition force they hailed from, known as Division 30.

Reuters has reported that one of the U.S.-trained Syrian fighters was killed in Friday’s attack. It was the first known battlefield casualty of the U.S. military’s program, which was launched in May and only fielded its first recruits in recent weeks.

The U.S. military’s training program has been challenged from the start, with many candidates being declared ineligible and some even dropping out.

U.S. President Barack Obama’s requirement that they target militants from ISIS 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 Pentagon, which had last week offered assurances that none of the U.S.-trained Syrian fighters had been captured, declined to publicly provide those assurances Monday. It also would not comment on the status of the U.S.-trained fighters.

“We’re not in a position to be able to provide you tactical details,” spokesman Capt. Jeff Davis told a news briefing, while acknowledging “developments” since the Pentagon’s assurances last week.

Davis said the United States remained committed to defending the Syrian fighters, including against the threat from Nusra Front, noting that the U.S. used airstrikes to help repel the attack by suspected Nusra Front fighters Friday.

The U.S. administration Monday said it was prepared to take “additional steps” to defend the U.S.-trained forces, warning President Bashar Assad’s regime “not to interfere.”

A U.S.-led coalition has provided air support for Kurdish and rebel militia fighting ISIS since September 2014, but has not struck regime positions.

White House spokesman Josh Earnest said Assad’s regime could be targeted if it attacked the new U.S.-backed forces. The United States, Earnest said, was “committed to using military force where necessary to protect the coalition-trained and equipped Syrian opposition fighters.”

There was no official reaction from the Syrian government Tuesday, but a political figure close to the regime told AFP that the U.S. had “relayed a message to Damascus not to worry about these statements.”

“It’s about hitting Al-Nusra hard, not the Syrian army,” he said.

Syrian Foreign Minister Walid al-Moallem arrived in Tehran Tuesday for talks with officials from allies Iran and Russia that are expected to focus on efforts to end the civil war in his country.

He was due to meet Mikhail Bogdanov, Russian President Vladimir Putin’s special envoy to the Middle East, later Tuesday evening before holding talks with Iranian Foreign Minister Mohammad Javad Zarif Wednesday, Iranian media reported.

Speaking about Moualem’s visit, Iranian deputy foreign minister Hossein Amir-Abdollahian suggested the chances of a diplomatic resolution of the war were growing. “Fortunately we see a change in the strategy of regional players in the Syrian crisis. If four years ago they believed war is the only solution, now they prefer to focus on diplomacy,” he was quoted as saying by Fars News Agency.