* Officials say cause of blast still unknown
* First Turkish citizens killed on border since October
* Area on Syrian side of crossing is rebel-controlled
A Syrian minibus exploded at a crossing on Turkey's border with Syria near the Turkish town of Reyhanli on Monday, killing at least 13 people including Turkish citizens and wounding dozens more, Turkish officials said.
Witnesses said they saw the vehicle drive up to the Cilvegozu 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 form 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 were due to fly to the area later on Monday to be briefed on the incident.
Prime Minister Tayyip Erdogan said the death toll from the blast had risen to 13, including three Turks. At least 28 people were wounded, 13 of them seriously.
The minibus exploded as it was driving through a stretch of no-man's land between the two countries only metres (yards) away from the Turkish border gate, where scores of Syrian civilians and Turkish humanitarian workers were congregated, Arinc said.
Television footage and photographs showed severe damage to a series of vehicles at the border, where a gate was blown open and part of the roof collapsed.
Turkey is a staunch supporter of the near two-year uprising against Syrian President Bashar al-Assad and has harboured both Syrian refugees and rebels. Violence has sometimes spilled over the border.
Five Turkish civilians were killed in October when a mortar shell hit a house in the Turkish border town of Akcakale.
Turkey also has responded in kind to gunfire and mortar rounds hitting its territory along the 910-km (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.
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.
Refugees cross back and forth and Turkish trucks also deliver goods into the no-man's land where they are picked up by Syrians.
Speaking from the scene of the blast, 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.
Alkhatib said on Sunday he was willing to hold talks with Assad's representatives in rebel-held areas of northern Syria to try to end a conflict that has killed some 60,000 people.
But others 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.
"Some say it's regime intelligence forces but we all know there are problems there between Jabhat al-Nusra and other brigades, particularly the (Free Syrian Army) Farouq Brigade," said one activist who declined to be named.
"There were tit-for-tat attacks before, so the options are open," he said.