A bomb blast Tuesday near a key oil field in eastern Syria targeted US-backed fighters but killed three workers instead, a war monitor said Tuesday.
The Syrian Observatory for Human Rights said the explosion occurred on the road outside the Al-Omar oilfield, the country's largest in the eastern province of Deir Ezzor.
Al-Omar is under the control of the US-backed Syrian Democratic Forces who have been fighting the Islamic State group.
Part of the oil field is used as an SDF base, while workers have resumed production in another.
Tuesday's blast struck on the nearby road "as a vehicle transporting oil field employees drove by, killing three workers," Observatory chief Rami Abdel Rahman said.
"The explosive device was detonated at a distance just after an SDF patrol had passed by," the head of the Britain-based monitoring group said.
There was no immediate claim for the blast, but IS often says it is responsible for deadly attacks in the area.
The Kurdish-led SDF expelled IS from its last patch of territory in the village of Baghouz south of Al-Omar in March, but has continued to sweep for jihadist sleeper cells.
IS overran large parts of Syria and neighbouring Iraq in 2014, declaring a "caliphate" and implementing its brutal interpretation of Islam on millions.
The jihadists seized Al-Omar that same year, before the SDF retook control a little more than a year later with backing from air strikes by a US-led coalition.
The air raids in 2015 destroyed the field, after the jihadists had reaped estimated oil revenues from it of between $1.7 million and $5.1 million a month, according to the coalition.
Syria's eight-year war has killed more than 370,000 people and displaced millions since it started in 2011 with the brutal repression of anti-government protests.
The conflict has seen the Damascus regime lose control of key oil fields and caused state hydrocarbon revenues to plummet by billions of dollars.