While Isil has largely been cleared from northern Syria, sleeper cells are known to operate and have carried out a number of insurgent attacks in recent months.
Special forces units are known to be on the ground in Syria, securing areas retaken from Isil, but officials have refused to comment on their activities.
Manbij is currently under threat of attack from Turkey, which stormed the nearby northern Syrian town of Afrin after an offensive against the Syrian Kurdish People’s Protection Unit (YPG) militia.
Ankara has repeatedly threatened to push its operations farther east to Manbij, which could lead to a direct confrontation between the two Nato allies.
The attack in Manbij happened late Thursday – the day US President Donald Trump said he would pull out forces “very soon”.
It came after Donald Trump caught his officials off guard by saying that America would be “out” of Syria “very soon” during a speech on infrastructure.
The US president dropped the hint while speaking in Ohio on Thursday night in an address intended to drum up support for more infrastructure spending.
He said: “We’re knocking the hell out of Isis. We’ll be coming out of Syria, like, very soon. Let the other people take care of it now.”