A Syrian warplane bombed the town of Ras al-Ain near the Turkish border for a third day on Wednesday as forces loyal to President Bashar al-Assad pressed an air assault to dislodge rebels.

The jet struck twice before circling and bombing again, rocking buildings on the Turkish side of the frontier and sending up huge plumes of smoke, a Reuters reporter in the Turkish border town of Ceylanpinar said.

It marked the third day of a jet bombardment that has brought Syria's civil war back perilously close to Turkish soil, testing a promise by Ankara to defend itself against any spillover of violence from its neighbour.

Rebels overran Ras al-Ain last week during an advance into Syria's mixed Arab and Kurdish northeast that has triggered some of the biggest refugee movements of the conflict.

Some 140 km (90 miles) to the west, three Turkish border villages were evacuated on Tuesday, a security official told Reuters, citing "security reasons".

Turkish media reports said the villages, located in the Suruc district of Sanliurfa province, had a combined population of around 1,000. The official declined to elaborate, but the move suggested the border clashes could be spreading.