Bomb blasts killed scores of people in the Syrian coastal cities of Jableh and Tartous on Monday, and wounded many others in the government-controlled territory that hosts Russian military bases, monitors and state media said.
Islamic State claimed responsibility for the attacks in the Mediterranean cites that have up to now escaped the worst of the conflict, saying it was targetting members of President Bashar al-Assad's Alawite minority.
The British-based Syrian Observatory for Human Rights said more than 120 people were killed. State media said 78 people died in the attacks on Assad's coastal heartland.
Attackers set off at least five suicide bombs and two devices planted in cars, the observatory said, the first assaults of their kind in Tartous, where government ally Russia maintains a naval facility, and Jableh in Latakia province, near a Russian-operated air base.
Fighting has increased in other parts of Syria in recent weeks as world powers struggle to revive a threadbare ceasefire and resurrect peace talks that collapsed in Geneva this year.
One of the four blasts in Jableh hit near a hospital and another at a bus station, while the Tartous explosions also targeted a bus station, the Observatory and state media reported.
Footage broadcast by the state-run Ikhbariya news channel of what it said were scenes of the blasts in Jableh showed several twisted and incinerated cars and minivans.
Pictures circulated by pro-Damascus social media users showed dead bodies in the back of pick-up vans and charred body parts on the ground.
The observatory said 53 people were killed in Jableh, and gave an earlier toll of more than 48 in Tartous.
State media put the total death toll at 78.
Islamic State claimed the attacks in a statement posted online by the group's Amaq news agency, saying its fighters had targeted "gatherings of Alawites".