The Syrian Observatory for Human Rights said the earth-shattering kaboom 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
...formerly ISIS or ISIL, depending on your preference. Before that they were al-Qaeda in Iraq, as shaped by Abu Musab Zarqawi. They're really very devout, committing every atrocity they can find in the Koran and inventing a few more. They fling Allaharound with every other sentence, but to hear western pols talk they're not
reallyMoslems....
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 bomb exploded at a distance just after an SDF patrol had passed by," the head of the Britannia-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.