Around 40 people have died and 100 are injured after two car bombs went off in a Turkish town near the Syrian border, according to officials.
Two leading cabinet members have promised action if "the usual suspects" in the Syrian military and intelligence services are proved to be behind the attack.
One of the car bombs exploded outside the city hall after 1pm today while the other went off 15 minutes later outside the post office in the town of Reyhanli, a main hub for Syrian refugees and rebel activity in Turkey's southern Hatay province, just across the border. Images showed people frantically carrying victims through the rubble-strewn streets to safety.
Deputy prime minister Bulent Arinc said: "We know that the Syrian refugees have become a target of the Syrian regime. Reyhanli was not chosen by coincidence."
"Our thoughts are that their mukhabarat (Syrian intelligence agency) and armed organizations are the usual suspects in planning and the carrying out of such devilish plans," he said.
Arinc said Turkey would "do whatever is necessary" if it is proven that Syria is behind the attack, and Foreign Minister Ahmet Davutoglu also vowed from Berlin that Turkey would act.
Prime minister Recep Tayyip Erdogan earlier raised the possibility that the bombings may be related to Turkey's peace talks with Kurdish rebels meant to end a nearly 30-year-old conflict.
Syrian mortar rounds have fallen over the 500-mile border before, but if the explosion turns out to be linked to Syria it would be by far the biggest death toll in Turkey related to its neighbour's civil war.
Turkey has been a crucial supporter of the Syrian rebel cause in the bloody civil war, now in its third year. Ankara has allowed its territory to be used as a logistics base and staging centre for Syrian insurgents.
The main Syrian opposition group, the Syrian National Coalition, condemned the "terrorist attacks" in Reyhanli, saying it stands together with the "Turkish government and the friendly Turkish people."
The coalition sees "these heinous terrorist acts as an attempt to take revenge on the Turkish people and punish them for their honorable support for the Syrian people," it said.
A third, small blast caused panic in the town hours later, but local reporters said it appeared to have been caused by a car engine or building boiler room.
Reyhanli is a center for aid and alleged weapon trafficking between Turkey and Syria, as well as for Syrian rebel activity. Apart from refugees living in camps, many Syrians escaping the civil war have also rented houses in the town.
The explosions came days before Erdogan is scheduled to travel to the US for talks, where Syria will be high up the agenda.
The war is thought to have killed more than 70,000 and displaced more than one million. It is fought between forces loyal to President Assad and rebels, mostly from the disparate Free Syrian Army.
The conflict has spilled periodically across the ceasefire line and Syria's borders with Lebanon, Iraq and Turkey, threatening to engulf the region.
The frontier area has seen heavy fighting between rebels and the Syrian regime. In February, a car bomb exploded at a border crossing with Turkey in Syria's Idlib province, killing 14. Turkey's interior minister has blamed Syria's intelligence agencies and its army for involvement.
Tensions flared between the Syrian regime and Turkey after shells fired from Syria landed on the Turkish side, prompting Germany, the Netherlands and the U.S. to send two batteries of Patriot air defense missiles each to protect their NATO ally.