A car bomb exploded on the Syrian side of the main Bab al-Hawa crossing with Turkey, killing at least seven and injuring 20, the Turkish news agency Dogan said.
The explosion on Tuesday occurred at a roadblock manned by brigades at the rebel-held crossing, several hundred meters from the Turkish side, Syrian opposition activists on the border said.
Pictures taken by Syrians on the scene showed several burned and mangled cars and an engine block that apparently had been blasted from the vehicle to the side of the road.
"The objective is to destabilize security at the crossing. The casualties are all civilians, from the people queuing to cross," activist Omar Aref said.
The explosion came a day after Turkey shot down a Syrian helicopter that had entered Turkish airspace.
The Syrian army said the shooting was "hasty" and accused the Turkish government, which backs the uprising against Syria's President Bashar al-Assad, of trying to escalate tensions along the border. No specific group was blamed for the attack.
Al Jazeera's Anita McNaught, reporting from Antakya in Turkey, said the blast signaled wider tensions at the border. Violence has often spilled over from the Syrian conflict into Turkey and Lebanon and sometimes into Jordan and Israel.
"The border is getting worse every month," she said. "There was a car bomb back in February that killed both Turks and Syrians. There was also a Syrian airstrike in April. "This is indicative of a worsening situation right across the border."
McNaught said the Turkish government is worried it will be left to handle Syria's worsening civil war alone and that this was one reason for its strong reaction to Monday's helicopter incursion.
Al Jazeera and wire services