Syrian authorities said the attack targeted three buses transporting workers from Al-Taim oil field, which is under Damascus regime control.

“The attack began with explosive devices that went off as the buses drove by, and then the group’s militants shot at them,” said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, a monitoring group in the UK with extensive sources inside Syria.

Despite the defeat of its so-called caliphate by US-backed Kurdish forces nearly four years ago, Daesh continues to claim attacks in Syria and across the border in Iraq.

The Kurdish-led Syrian Democratic Forces launched a new offensive against Daesh on Thursday, following a recent militant attack on a prison in Raqqa, northwest of the attack on the bus.

It said it had arrested 52 Daesh “mercenaries and facilitators” in residential areas on the first day of the operation.

The SDF, which regularly launches attacks against the militants, said its latest offensive aimed to eliminate Daesh from areas that had been “the source of the recent terrorist attacks.”

In addition to the thwarted Raqqa prison attack, the SDF said Daesh fighters had recently carried out eight assaults in the Deir Ezzor area, Hasakeh and the Al-Hol camp for displaced people, which houses family members of Daesh militants.

On Monday, the SDF said six Kurdish fighters were killed when Daesh attacked a security complex in Raqqa, the militants’ former de facto capital in Syria, in an attempt to free imprisoned terrorists.

It said Daesh was trying to “take advantage” of recent Turkish airstrikes on Kurdish forces in northeast Syria.

Turkey supports rebels in Syria’s northern border zone, but opposes Syria’s Kurds, which it sees as linked to Kurdish separatists at home.