An attack in eastern Syria killed 12 oil field workers, a war monitor said on Friday. The British-based Syrian Observatory for Human Rights, which draws on extensive sources inside Syria, gave the toll of a dozen deaths in the assault near an oil field west of Deir Ezzor. It blamed cells linked to the Islamic State group (IS). Syria’s state news agency SANA gave a toll of 10 dead in the “terrorist attack that targeted three buses transporting workers” from al-Taim oil field, which is under Syrian government control. Despite the defeat of its “caliphate” in Syria by US-backed Kurdish forces nearly four years ago, IS continues to claim attacks in Syria and across the border in Iraq. “The attack began with explosive devices that went off as the buses drove by, and then the group’s militants shot at them,” Observatory director Rami Abdel Rahman told AFP. On Thursday the Kurdish-led Syrian Democratic Forces (SDF) said they had begun an offensive against IS, following their assault on a prison in Raqa, northwest of the attack on the bus. The SDF, which regularly launches operations against the IS, said its latest offensive aimed to eliminate IS from areas that had been “the source of the recent terrorist attacks”. It said it was carrying out the operation alongside a US-backed anti-IS coalition, although the international force did not immediately confirm its participation.