Attacks in eastern Syria killed 12 oil field workers and a Kurdish fighter, a war monitor said Friday, a day after Syrian Kurdish-led forces announced an offensive against jihadists.
The Britain-based Syrian Observatory for Human Rights, which draws on extensive sources inside Syria, gave the toll of a dozen dead in one assault near an oil field west of Deir Ezzor.
In a separate ambush near Al-Shaddadi, south of the northeastern city of Hasakeh, a fighter of the Kurdish-led Syrian Democratic Forces (SDF) was killed and five more wounded, the Observatory said.
It blamed cells linked to the Islamic State group, which have previously carried out attacks in both areas.
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.
"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.
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.
On Thursday, the SDF said it had begun an offensive against IS, following a recent assault by the jihadists on a prison in their onetime bastion of Raqa.
The SDF, which regularly launches operations against the jihadists, 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-led anti-IS coalition, although the international force did not immediately confirm its participation.
In addition to the thwarted Raqa prison attack, the SDF said IS 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 IS militants.
The SDF said on Friday that 52 IS "mercenaries and facilitators" had been arrested in residential areas during its newly launched operation.
On Monday, the SDF said six Kurdish fighters were killed when IS attacked the prison in Raqa in a bid to free jailed comrades.
Referring to recent Turkish air strikes on Kurdish forces in the northeast, the SDF said IS was trying to "take advantage" of the situation.
Turkey backs rebels in Syria's northern border zone, but opposes Syria's Kurds, which it sees as inextricably linked to Kurdish "terrorists" at home.
After IS seized vast areas of Iraq and Syria in 2014, it ruled with brutality until local forces backed by the US-led coalition defeated them, first in Iraq in late 2017 and then in Syria in March 2019.
In January, jihadists killed nine Syrian soldiers and allied militiamen near oil installations on the edge of Deir Ezzor, the Observatory said at the time.
That was about a month after an attack on a bus carrying oil workers reportedly killed at least 10.