International diplomats said the conflict, which could engulf Lebanon, Jordan and Iraq could give cover to Syrian President Bashar Assad to crack down more harshly on an insurgency seeking his ouster.
Turkish residents near Syria were evacuated Monday and Prime Minister Recep Tayyip Erdogan said his country must be prepared as if it will fight a war.
Police ordered residents near the Turkish-Syrian border fence to move out after Syrian mortar fire landed on Turkish territory for a fifth straight day. Two Turk women were killed and eight others injured when Syrian shells landed in a residential neighborhood in the border city Akcakale, Anatolian News Agency reported.
Akcakale is where Syrian artillery killed five Turkish citizens Wednesday, escalating tensions between the two former allies.
Turkish troops immediately retaliated Sunday, hitting an ammunition depot belonging to the Syrian army, the Hurriyet Daily News reported.
Witnesses said they saw a fire break out.
"You have to be prepared as if you are about to be in a war at any time," Erdogan said in Istanbul Sunday in response to opposition criticism to his military response to Syria.
"If you cannot be prepared, you cannot call yourself a powerful country and powerful nation," he said.
Turkey has fired artillery shells across the border every day since Wednesday in response to Syrian artillery hitting towns along its border. Monday marked the sixth such day.
Assad's regime has refused to apologize for Wednesday's deadly shelling. It says it first wants to investigate the source of the fire, which it said could be from rebel fighters.
In Syria's al-Raqqa province, across from Akcakale, rebel fighters said Sunday's shelling on Turkey came from regime tanks in the village of Ain Arous, about 7 miles from the border.
The Daily News said Tel Abyad, a Syrian town about 9 miles from Turkey's border that was the focus of intense retaliatory shelling from Turkey, was recently seized by Syrian rebels. The Wall Street Journal said Tel Abyad was being shelled by Syrian army positions seeking to recapture it.
Notable deaths of 2014 [PHOTOS]