An attack in eastern Syria killed 12 oil field workers, a war monitor said on Friday, a day after Syrian Kurdish-led forces announced an offensive against jihadists.
The British-based Syrian Observatory for Human Rights, which draws on extensive sources inside Syria, gave the toll of a dozen dead in the assault near an oil field west of Deir al-Zor.
It blamed cells linked to the Islamic State group (IS). The jihadists have previously carried out attacks in the area, and a similar deadly assault took place last year.
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 a recent jihadist assault on a prison in Raqqa, northwest of the attack on the bus.
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-backed anti-IS coalition, although the international force did not immediately confirm its participation.
In addition to the thwarted Raqa prison attack, SDF said IS fighters had recently carried out eight assaults in the Deir al-Zor area, Hasakeh and the al-Hol camp for displaced people, which houses family members of IS militants.
The SDF on Friday said 52 IS "mercenaries and facilitators" had been arrested in residential areas during its newly launched operation.
Taking 'advantage'
On Monday, the SDF said six Kurdish fighters were killed when IS attacked a security complex in Raqqa, the jihadists' former de facto capital in Syria, in a bid to free imprisoned militants.
Referring to recent Turkish airstrikes on Kurdish forces in northeast Syria, 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.
More than a year later the SDF, which spearheaded the fight against IS in Syria, drove the group from Baghuz, its last stronghold there.
IS said Monday's attack on Raqqa aimed to avenge "Muslim prisoners" and female relatives of jihadists in al-Hol camp.
Pro-Iranian militias also have significant influence in the Syria-Iraq border region near where the latest attack took place.
The Observatory blamed "a base of pro-Iranian militias" for a November 17 rocket strike that targeted the US-led coalition's Green Village base, roughly 50 kilometers from the bus attack.
Green Village is in Syria's largest oil field, Al-Omar. There were no reported injuries in that incident.
In January jihadists killed nine Syrian soldiers and allied militiamen near oil installations on the edge of Deir al-Zor, 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.
Syria's war began with the brutal repression of peaceful protests in 2011 and escalated to pull in foreign powers as well as the jihadists.
Russia's military intervention in 2015 helped turn the tide in favor of Syria's President Bashar al-Assad.
On Wednesday defense ministers from Russia, Turkey and Syria met in Moscow for their first such talks since the Syrian conflict started, the Russian defense ministry said.
Nearly half a million people have been killed in Syria's war, which forced around half of the country's pre-war population from their homes.