"We can confirm that Mohammad Haydar Zammar, a Syrian-born German national, was captured more than a month ago by SDF partners," Pentagon spokesman Eric Pahon said in a statement, referring to the Syrian Democratic Forces (SDF).

A senior Kurdish commander told AFP the previous day that Zammar had been detained "and is now being interrogated."

Zammar, in his mid-fifties, has been accused of recruiting some of the September 11 Al-Qaeda hijackers who carried out attacks on Washington and New York, killing almost 3,000 people.

He was detained in Morocco in December 2001 in an operation involving CIA agents, and was handed over to the Syrian authorities two weeks later.

A Syrian court sentenced Zammar to 12 years in prison in 2007 for belonging to the Muslim Brotherhood, a charge that at the time could have resulted in the death penalty.

But conflict broke out in Syria four years later, and many hardline Islamist prisoners were released from jail or broke free and went on to join jihadist groups fighting in the war.

Al-Qaeda operated a branch in Syria known as Al-Nusra Front, but the affiliate has since claimed to have broken off ties.

The Islamic State jihadist group also rose to power in the country's north and east, but a US-backed alliance has ousted it from swathes of its onetime "caliphate."

The SDF, a coalition of Arab and Kurdish fighters, has caught several foreign members of IS in Syria in recent months, particularly since it captured the northern city of Raqa from the jihadists.

The Kurdish commander who spoke to AFP on Wednesday declined to say whether Zammar had been actively fighting as a member of an extremist group in Syria.