AMMAN—Twenty-eight people were killed in an attack on a bus along a main highway in Syria’s Deir al-Zor province that borders Iraq, Syrian state media said on Wednesday, an incident that residents and defectors say was an ambush on an army vehicle. There were no more details of the incident in an area where mostly Syrian army and Iranian-backed militias are based, near the ancient city of Palmyra. A senior military defector in the area said the vehicle carried soldiers and pro-government militias who had finished their leave and were on their way back to their base in the desolate, sparsely populated area. Another source said at least thirty soldiers were killed, mostly from the Syrian army’s elite Fourth Brigade, which has a strong presence in the rich oil-producing province since fighters with the ISIS terrorist group were ousted at the end of 2017. Deir al-Zor residents and intelligence sources …