For the fourth straight day, Turkey launched mortars and artillery into Syrian territory in response to Syrian shelling of Turkish soil.

Reuters:

Turkey returned fire after mortar bombs shot from Syria landed in a field in southern Turkey on Saturday, the day after Prime Minister Tayyip Erdogan warned Damascus Turkey would not shy away from war if provoked.

It was the fourth day of Turkish strikes in retaliation for mortar bombs and shelling by Syrian forces that killed five Turkish civilians further east on Wednesday.

The strikes and counter-strikes are the most serious cross-border violence in Syria's conflict, which began as a democracy uprising but has evolved into a civil war with sectarian overtones. They highlight how the crisis could destabilize the region.

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 more than 30,000 people have died, according to the United Nations.

Turkey has nearly 100,000 Syrian refugees in camps on its territory, has allowed rebel leaders sanctuary and has led calls for Assad to quit. Its armed forces are far larger than Syria's.

Erdogan said on Friday his country did not want war but warned Syria not to make a "fatal mistake" by testing its resolve. Damascus has said its fire hit Turkey accidentally.

Foreign Minister Ahmet Davutoglu struck a more defensive tone on Saturday, saying parliament's authorization of possible cross-border military action was designed as a deterrent.

"With the mandate we did not take a step towards war, we showed the Syrian administration our deterrence, making the necessary warning to prevent a war," he said.

"From now on, if there is an attack on Turkey it will be silenced," he said in an interview with state broadcaster TRT.

Neither side wants a general war, but once begun, military action has a nasty way of getting out of control. This is exactly the sort of fallout from the Syrian civil war that policy makers in Washington have been fearing for weeks and it will take cool heads in both Ankara and Damascus to keep the fighting from escalating.