The Turkish government says 13 people were killed and 28 wounded when a vehicle – thought to be a van or a minibus – blew up at a border crossing between Turkey and Syria.
The deputy prime minister said all possible causes for the blast were being investigated, including a suicide attack, but it was wrong to jump to hasty conclusions.
Other possible causes being given are a mortar attack or a petrol tank exploding.
The mayor of Reyhanli, the local town, said the vehicle had Syrian licence plates. It is thought to have arrived from Syria.
The victims are said to be from both nationalities: the majority of the dead are thought to be Syrian, with three Turkish people killed.
The border gate at Cilvegozu near Reyhanli is opposite the Syrian gate of Bab al-hawa which the rebels captured last July.
Witnesses say the vehicle was parked in a car park. According to officials the blast caused several other vehicles to catch fire.
Refugees cross from one side to the other and Turkish trucks also deliver goods into no-man’s land between the two gates, where they are picked up by Syrians.
Turkey is a strong supporter of the two-year uprising against President al-Assad’s government and has sheltered both refugees and rebels.