Turkey has said it believes fighters loyal to Syrian President Bashar al-Assad were behind twin car bombings that killed 46 people in a Turkish border town.
Foreign Minister Ahmet Davutoglu said those involved in the bombings in Reyhanli were thought also to have carried out an attack on the Syrian coastal town of Banias a week ago.
Fighters backing Assad were reported to have killed at least 62 people in Banias.
Syrian Information Minister Omran Zubi denied any Syrian involvement and rejected what he called "unfounded accusations".
The Syrian conflict, now in its third year, has inflamed a confrontation between Sunni and Shia Muslims in the Middle East.
Shia Iran supports president Assad, and Sunni powers like Saudi Arabia back the rebels.
Banias is a Sunni pocket in the midst of a large Alawite enclave on Syria's Mediterranean coast, and activists in the area accuse militias loyal to Assad, an Alawite, of ethnic attacks.
Reyhanli also has a significant Sunni population, including thousands of Syrians, and has become a logistics base for the rebels fighting Assad just over the border.