Turkey returned fire after Syrian mortar bombs landed in a field in southern Turkey on Saturday, the day after Prime Minister Tayyip Erdogan warned Syria that Turkey would not shy away from war if provoked.
It was the fourth day of Turkish retaliation for firing by Syrian forces that killed five Turkish civilians on Wednesday.
The exchanges 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, according to the United Nations, more than 30,000 people have died.
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.
Turkish Foreign Minister Ahmet Davutoglu said on Saturday that parliament's authorization of possible cross-border military action was designed as a deterrent.
"From now on, if there is an attack on Turkey it will be silenced," he said in an interview with state broadcaster TRT.
Western powers have backed fellow NATO member Turkey over Syria but have shown little appetite for the kind of intervention that helped topple Libya's Muammar Gaddafi. Turkish calls for a safe zone in Syria would require a no-fly zone that NATO states are unwilling to police.
'Virtually impossible'
Davutoglu said international mediator on Syria Lakhdar Brahimi would come to Turkey before Russian President Vladimir Putin visits Ankara within the next 10 days.
Arab League chief Nabil Elaraby in a newspaper interview called Brahimi's Syria mission "virtually impossible." Asked about the efforts of the Egypt-Saudi-Turkey-Iran quartet to solve the crisis, Elaraby said, "The solution must comprise Iran. The important thing is that matters get moving."
The 18-month-old Syrian revolt increasingly pits a Sunni Muslim opposition against Assad's Alawite minority, an offshoot of the Shia Islam that predominates in Iran, whose government backs Assad's government.
Rebels in the Syrian city of Aleppo said government troops tried to storm the Sakhour district on Saturday but were pushed back after heavy clashes. Activists across Syria said there was fighting in several cities and towns including the central city of Homs and in Damascus countryside.
The British-based Syrian Observatory for Human Rights said at least 60 people, including 36 government soldiers, were killed in clashes across the country on Saturday.
Syrian rebel forces are riven by divisions but Syrian government forces appear to lack the numbers to land a knockout blow and permanently hold rebellious areas.
US President Barack Obama on 25 September accused Iran of helping keep Assad in power but has refused to arm Syria's rebels, partly for fear some of those fighting Assad's rule are Islamist radicals equally hostile to the West.
US Defense Secretary Leon Panetta expressed renewed worry on Saturday that the hostilities in Syria could spread.
"Whether or not that conflict begins to extend into the neighboring countries such as Turkey remains to be seen. But obviously the fact that there are now exchanges fired between these two countries raises additional concerns that this conflict could broaden," Panetta told a news conference in Lima with Peru's defense minister.
Iran on Saturday called for the immediate release of Iranians held captive by Syrian rebels and said it would hold the rebels and their supporters responsible for their lives.
Syrian rebels seized a busload of 48 Iranians in early August on suspicion of being military personnel. Tehran says they were pilgrims visiting a Shi'ite shrine in Damascus.
Mortars land in Turkey
At least three rounds fired from Syria landed inside Turkey's Yayladagi district on Saturday, the office of the governor of the Turkish province of Hatay said.
It said the fire appeared to have been aimed by Syrian forces at rebels along the border. There were no casualties. Turkish border troops fired back mortars in response.
There were two similar incidents in Hatay on Friday, when Erdogan issued his warning.
"Those who attempt to test Turkey's deterrence, its decisiveness, its capacity, I say here they are making a fatal mistake," he said in a bellicose speech to a crowd in Istanbul.
"We are not interested in war, but we're not far from war either. This nation has come to where it is today having gone through intercontinental wars," he said.
Turkish artillery bombarded Syrian military targets on Wednesday and Thursday, killing several Syrian soldiers after Syria's initial fatal bombardment. The UN Security Council condemned the original Syrian attack.
Russia, a staunch ally of Syria, said it received assurances from Damascus the strike on Turkey was an accident but Erdogan dismissed them, saying Syrian fire had repeatedly hit Turkey.
Wednesday's Syrian strike on the town of Akcakale was of a different magnitude to previous incidents, a Turkish official told Reuters.
"Wednesday was different. There were five or six rounds into the same place. That's why we responded a couple of times, to warn and deter. To tell the (Syrian) military to leave. We think they've got the message and have pulled back from the area."
Syria has since ordered its warplanes and helicopters not to go within 10 kilometers (six miles) of the Turkish border and artillery units not to fire shells close to the border, according to Turkish broadcaster NTV. Syria has not confirmed this.
Turkey's state-run Anatolian news agency said a large number of Turkish troops had been sent to the Oncupinar border area of Kilis province.
