A car has exploded at a border crossing between Turkey and Syria, killing at least 12 people and wounding dozens more.
The mayor of Reyhanli, the nearest town to the border checkpoint, told CNN Turk that four of those killed were Turkish and that the car which exploded had Syrian licence plates.
Witnesses said they saw it drive up to the border post, one of the main crossing points for Syrian refugees into Turkey, shortly before the explosion.
Deputy Prime Minister Bulent Arinc said the government was investigating all possible causes for the blast, including a suicide attack, but it was wrong to jump to hasty conclusions.
"The information that we have for now is that a minibus with Syrian number plates coming from the other side exploded. It was a powerful explosion," Arinc, who is also the government spokesman, told reporters after a regular cabinet meeting.
"But whether this was a vehicle laden with explosives or another type of explosion, I think, at the latest, will become clear tomorrow," he said.
Turkey's interior, justice and customs ministers are due to fly to the area to be briefed on the exposion.
Arinc said the death toll from the blast had risen to 12, including three Turks and nine Syrians. At least 28 people were wounded, 13 of them seriously.
Television footage and photographs showed severe damage to a series of cars at the border, where a gate was blown open and part of the roof collapsed.
Prime Minister Tayyip Erdogan interrupted a cabinet meeting to be briefed on the explosion, Turkish media reported.
Turkey is a staunch supporter of the almost two-year-old uprising against Syrian President Bashar al-Assad and has harboured both Syrian refugees and rebels.
Five Turkish civilians were killed in October 2012 when a mortar shell hit a house in the Turkish border town of Akcakale.
Turkey has also responded in kind to gunfire and mortar rounds hitting its territory along the 910km (565 mile) border and is hosting six NATO Patriot missile batteries meant to defend it against attacks from Syria.
Tensions have increased in recent weeks after NATO said it had detected launches of short-range ballistic missiles inside Syria, several of which have landed close to the Turkish border.
Turkey has scrambled warplanes along the frontier, fanning fears the war could spread and further destabilise the region.
Speaking from scene of the explosion, Syrian opposition campaigner Osama Semaan said he believed the explosion was an attack by Assad loyalists in response to an offer of talks from opposition Syrian National Coalition leader Moaz Alkhatib.
"It seems it is a deadly message from the Syrian regime after Sheikh Moaz offered it to talk in northern Syria," Mr Semaan said.
The Cilvegozu border gate, several kilometres outside the town of Reyhanli, sits opposite the Syrian gate of Bab al-Hawa, which the rebels captured last July.
Refugees cross back and forth and Turkish trucks also deliver goods into no-man's land between the two gates, where they are picked up by Syrians.