The Syrian government has accused Turkey, Qatar and Saudi Arabia of being behind a wave of bombings in the coastal cities of Tartous and Jableh, the BBC reports.
The state-run news agency Sana said the attacks constituted a “serious escalation”, and were aimed at derailing peace efforts.
State media said at least 78 people were killed, while a monitoring group put the death toll at more than 145.
So-called Islamic State (IS) has said it was behind the attacks.
The cities, on Syria’s Mediterranean coast, are part of the heartland of President Bashar al-Assad and have, until now, escaped the worst of the civil war.