The explosion occurred at a roadblock manned by extremist militants at the entrance of the militant-held crossing, several hundred meters from the Turkish side, local reports said.
Pictures taken by locals on the scene showed several burnt and mangled cars and an engine block that apparently flew to the side of the road from the vehicle.
"The objective is to destabilize security at the crossing. The casualties are all civilians, from the people queuing to cross," said Omar Aref, one of the observers.
It was not clear who was behind the attack.
The explosion came a day after Turkey shot down a Syrian helicopter that Turkish officials claimed violated their airspace.
The Syrian army criticized Turkey’s "hasty" action which left the two pilots of the chopper dead and said Ankara was trying to escalate tensions along the border.
Syria has been struggling with new foreign threats amid the massive insurgency that has gripped the country for more than two and a half years.
A US plan to bomb the country over a chemical attack near Syrian capital on August 21 was canceled with Russia’s mediation that sought to ease the tensions by putting Syria’s stockpile of chemical weapons under international control.
The offer was widely welcomed both in Syria and other countries that were concerned by US military ambitions and its regional consequences.
SHI/SHI