WASHINGTON/ISTANBUL – Ankara returned fire after Syrian mortar bombs landed in a field in southern Turkey on Saturday.

Saturday was the fourth day of Turkish retaliation for firing by Syrian forces that killed five Turkish civilians on Wednesday.

On Friday, Prime Minister Tayyip Erdogan warned Damascus that Turkey would not shy away from war if provoked.

The exchanges are the most serious cross-border violence in Syria’s conflict. They highlight how the crisis could destabilize the region.

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

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.

The United States expressed strong support for Turkey Friday as it took military action to respond to Syrian attacks on its border.

“We do certainly stand behind Turkey as they take that action, because we believe that action is appropriate,” White House spokesman Josh Earnest said when asked about the flare-up between the two countries.

NATO has passed a resolution condemning the violence, as did the UN Security Council.

But UN Security Council permanent member and Syria ally Russia has called for restraint on the part of Turkey, as has Iran.

Earnest referred to Turkey’s actions as “designed to ensure that their sovereignty is no longer violated by Syrian aggression.”

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 called Brahimi’s Syria mission “virtually impossible,” in a newspaper interview.

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.”

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 countryside near Damascus.

The British-based Syrian Observatory for Human Rights said at least 60 people, including 36 government soldiers, were killed across the country on Saturday in clashes.

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.

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.

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 km. 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.

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