The Turkish military returned fire after a mortar bomb shot from Syria landed in countryside in southern Turkey, the state-run Anatolian news agency reported.

It was the latest in a series of Turkish retaliatory attacks in response to mortar bombs and shelling by Syrian forces that have killed five Turkish civilians.

The strikes and counter-strikes have been the most serious cross-border violence seen so far in Syria's conflict.

Turkish Prime Minister Tayyip Erdogan said yesterday his country did not want war but warned Syria not to make a "fatal mistake" by testing its resolve.

Damascus has said the mortars landed in Turkey accidentally.

Anatolian said the mortar round hit countryside near Guvecci village in the Yayladagi area amid intense clashes on the Syrian side of the border in Idlib province.

"Military units on the Turkish border launched retaliatory fire immediately," the agency said, without identifying its source or mentioning any casualties.

NATO member Turkey was once an ally of Syrian President Bashar al-Assad but turned against him after his violent response to an uprising in which, according to the United Nations, so far more than 30,000 people have died.

Turkey has nearly 100,000 Syrian refugees in camps on its territory.

It has allowed rebel leaders sanctuary and has led calls for Assad to quit.

Turkey's armed forces are far larger than Syria's.