Turkey has once again returned fire after a mortar bomb fired from Syria landed in the countryside on Saturday morning.

The state-run Anatolian news agency reported the latest in a series of Turkish retaliations in response to shelling by Syrian forces. On Wednesday, five Turkish civilians were killed by a stray shell further east along the border.

The increase in cross-border fire is the most serious escalation so far of the increasingly tense stand off between the Syrian government and former ally Turkey, and a clear sign of the strain on the stability of the region at large.

Turkish Prime Minister Tayyip Erdogan said on Friday his country was not looking for a war but warned Syria not to make a "fatal mistake" by testing its resolve. Damascus insisted at the time that the mortars landed in Turkey accidentally and apologised.

Anatolian said Saturdays' mortar round hit open ground near the village of Guvecci in the Yayladagi area, as intense firefights were being fought over the nearby border.

"Military units on the Turkish border launched retaliatory fire immediately," the agency said, without giving any details on their sources, or any casualties.