At least 101 people were killed in a spate of bombings in two Damascus regime bastions on the Mediterranean coast, the Syrian Observatory for Human Rights said.
Fifty-three people were killed in the city of Jableh and another 48 died in Tartus further south, according to the Syrian Observatory for Human Rights monitoring group.
Observatory head Rami Abdel Rahman said they were “without a doubt the deadliest attacks” on the two cities since Syria’s conflict erupted in March 2011.
The Islamic State group claimed the attacks via its Amaq news agency, saying IS fighters had attacked “Alawite gatherings” in Tartus and Jableh.
Seven bombs – most of them suicide attacks – hit Jableh and Tartus almost simultaneously on Monday morning.
Syrian state media also reported the attacks but gave a total of 78 dead, including 45 in Jableh and 33 in Tartus.
State television broadcast footage of a bus station that was hit by one blast in Tartus.
Charred minibuses lay on their sides while others were still ablaze.
A Facebook page sharing local news from Jableh, where another bus station was targeted, as was a government hospital, shared footage of people around fire trucks near several bombed-out cars.
Both cities are strongholds of the regime of President Bashar al-Assad, whose family hails from the village of Qardaha, just 25 kilometres east of Jableh.
They have been relatively insulated from the war raging in Syria, which has killed at least 270,000 people since March 2011.