The minibus, bearing Syrian number plates, exploded at a crossing on the border near the Turkish town of Reyhanli on Monday, killing 14 people and wounding dozens more. "This is a terrorist act. We are working on several alternatives. It was probably carried out by a Syrian," Turkish Interior Minister Muammer Guler told reporters in parliament.
A statement by the Syrian National Council (SNC), an exile opposition group, said the bombing occurred as a delegation comprising the SNC leadership was expected to pass through the crossing on its way back from the Syrian city of Aleppo.
It said "Syrian regime tools" carried out the bombing, describing it as the latest of the "regime crimes".
Turkish police investigations were focusing on three people seen leaving the minibus before the explosion. The minibus was parked in a waiting area at the Cilvegozu border post, one of the main crossing points for humanitarian aid into Syria, before it exploded, according to Turkey's Deputy Prime Minister Besir Atalay.
"Amid the bustle, a Syrian-plated vehicle arrives, waits for some time and three people slip away. Twenty minutes later the vehicle exploded," the minister said. "There is footage and prosecutors and police are working on that," he said, declining to speculate on who was responsible.
Two of the three people left in a car heading into Syria and the third person slipped away into Turkey, media reports said.
One activist said the blast could have been the work of one of several factions within Syria's armed opposition coalition, including the al Qaeda-linked Jabhat al-Nusra, one of the most coherent and disciplined anti-Assad forces in Syria.
Turkey's interior, justice and customs ministers flew to the area late on Monday to be briefed on the incident.
The customs minister told a news conference security would be increased at the border but humanitarian aid would continue to be sent across, state-run Anatolian news agency reported.
At least 28 people were wounded in the attack. The blast damaged to scores of vehicles at the border post, where a gate was blown open and part of the roof collapsed.
Turkey is a staunch supporter of the near two-year uprising against Assad and has harboured both Syrian refugees and rebels. Violence has sometimes spilled over the border.
Tensions 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 war planes along the frontier, fanning fears the war could spread and further destabilise the region.
The Cilvegozu border gate, several km (miles) outside Reyhanli in Turkey's Hatay province, sits opposite the Syrian gate of Bab al-Hawa, which the rebels captured last July.