The Syrian Observatory for Human Rights said overnight shelling across the frontier by Turkish troops hit a Syrian army post near the border town of Tel Abyad. It gave no exact figure for the number of soldiers killed.

Turkey has demanded that the UN Security Council take action against Damascus after Wednesday's incident, in which mortar fire from Syria killed five Turkish civilians, including a mother and her three children, in the Turkish town of Akcakale.

Turkish artillery fired retaliatory salvos back at Syrian targets during the night, marking a sharp escalation in the crisis between the neighbours.

The artillery fire resumed again on Thursday morning, a Turkish security source told AFP on condition of anonymity.