"An explosive device detonated as a bus carrying oil facility guards affiliated with the defence ministry passed by, killing five of them and wounding 13 others, including civilian bystanders," the official said, requesting anonymity.

State television said a blast hit a bus on the road between the cities of Deir Ezzor and Mayadeen, some 50 kilometres (30 miles) away.

No group immediately claimed responsibility for the attack, but the Britain-based Syrian Observatory for Human Rights said the perpetrators were "likely affiliated with an Islamic State (IS) group cell".

IS jihadists, once in control of large swathes of Iraq and Syria, were territorially defeated in Syria in 2019 in a battle spearheaded by the US-backed, Kurdish-led Syrian Democratic Forces (SDF) with support from an international coalition.

The jihadists still maintain a presence, particularly in Syria's vast desert, launching attacks mostly on Kurdish-controlled areas in the country's northeast.

During Syria's civil war, which erupted in 2011, IS carried out similar attacks on buses targeting the forces of former ruler Bashar al-Assad.

Since the new Islamist-led authorities took power after Assad's December ouster, jihadist attacks on government-controlled areas have been scarce.

In May, IS claimed its first attack on the new forces, with the Observatory saying one member of Syrian army personnel was killed and three others wounded.

The following month, authorities accused IS of being behind a deadly suicide attack in a Damascus church that killed 25 people, though the group never claimed responsibility.