An attack in eastern Syria killed 10 oil field workers, state news agency SANA reported on Friday, a day after Syrian Kurdish-led forces announced an offensive against jihadists.
In addition to the nearly dozen dead, "two others have been wounded in a terrorist attack that targeted three buses transporting workers from al-Taim oil field in Deir Ezzor" province, SANA reported.
It did not provide any information on the nature of the attack in the Kurdish-held area or who may be behind it, but a British-based war monitor said "cells of the Islamic State group" carried out the assault near the oil field.
"The attack began with explosive devices that went off as the buses drove by, and then the group's militants shot at them," Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, told AFP.
On Thursday the Kurdish-led Syrian Democratic Forces said they had begun an offensive against Islamic State (IS) group fighters, following an earlier jihadist assault on a prison in Raqa, northwest of the attack on the bus.
The Syrian Democratic Forces (SDF) said the offensive, dubbed "Operation al-Jazeera Thunderbolt", aimed to "eliminate" IS fighters from areas that had been "the source of the recent terrorist attacks".
The SDF said it was carrying out the operation alongside the US-backed coalition, although there was no immediate confirmation from the international force that they were taking part.
The SDF statement said that in addition to the thwarted Raqa attack, 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.
On Monday, six Kurdish fighters were killed when IS militants attacked the complex in Raqa, the jihadist group's former de facto capital in Syria, in a bid to free fellow militants imprisoned there.
Referring to recent Turkish airstrikes on Kurdish forces in northeast Syria, the SDF said IS was trying to "take advantage" of the situation by "carrying out more terrorist attacks".
After a meteoric rise in Iraq and Syria in 2014, IS saw its so-called caliphate collapse, but fighters remain.
Supported by an international anti-jihadist coalition led by the United States, the SDF spearheaded the fight against IS in Syria and drove the group from its last stronghold in the country in 2019.
IS continues to claim attacks in Iraq and Syria, and the SDF regularly launches operations against the jihadists.
IS said Monday's attack on Raqa aimed to avenge "Muslim prisoners" and female relatives of jihadists living in Al-Hol camp.
This was the most significant jihadist attack on a prison since IS fighters launched their biggest assault in years in January, when they attacked the Ghwayran prison in the Kurdish-controlled city of Hasakeh.