An estimated 148 people were killed on Monday as multiple car bomb and suicide attacks hit civilian targets in two usually peaceful cities on Syria's government-held Mediterranean coast, the Syrian Observatory for Human Rights reported.
Supporters of the Islamic State group circulated a claim of responsibility for the attacks in Tartus and Jableh on social media, although dpa could not independently verify it.
The attacks are the worst in decades in the coastal region, which is firmly under government control and hosts hundreds of thousands of people fleeing conflict in inland provinces.
Analyst Charles Lister of the US-based Middle East Institute warned that the attacks appeared aimed at increasing sectarian tension, along the lines of deadly attacks claimed by Islamic State in neighbouring Iraq.
Many locals in the region belong to President Bashar al-Assad's Alawite religious minority, while most of the displaced people are Sunni Muslims from areas where government forces have been clashing with mainly Sunni rebels and jihadists.
"While the veracity of a purported ISIS claim of responsibility cannot be substantiated, the perpetrators have one clear objective: to stoke a self-sustaining cycle of sectarian tit-for-tat violence that incapacitates moderates and empowers extremists," Lister said.
Locals and regime loyalists attacked a camp for displaced people in Tartus shortly after the bombings, the Observatory reported.
The Britain-based monitoring group said security forces arrested tens of refugees in the area originating from conflict-hit mainly Sunni provinces in inland Syria.
"Some loyalists to the regime also burnt tents of refugees in the al-Karnak camp and insulted refugees living there," Observatory director Rami Abdel-Rahman said, adding that he was concerned about the safety of the refugees.
Monday's attacks began when a car bomb exploded in a bus and taxi station in Jableh, quickly followed by a suicide attacker blowing himself up, according to the organization.
Two more suicide attackers then blew themselves up at an electricity company office and in the entrance to a hospital emergency department.
Syrian state television said that one of the bombers in Jableh was apparently helping the wounded when he blew himself up.
Minutes after the first bombings in Jableh, a car bomb went off in the bus and taxi station in Tartus, 50 kilometres further south, the Observatory said. Two suicide bombers then blew themselves up amid groups of people in the city.
Nadim Houry, deputy Middle East director at Human Rights Watch (HRW) said "deliberately targeting civilians is a war crime, and anyone involved in preparing, ordering, or carrying out such a crime should be held accountable."
"While it may be impossible to influence the criminal behavior of ISIS, Syria’s multiple warring sides and the international parties that support them can and should do more not only to condemn such ISIS attacks but also to ensure that they stop targeting civilians," Houry said.
Pictures aired by state television showed heavy damage in the areas where the blasts took place and pools of blood covering the streets.
Other television footage showed minibuses, vans and taxis ablaze, while panicked people ran and shouted for help.
State news agency SANA put the death toll at 45 in Jableh and 33 in Tartus.
Islamic State supporters on social media circulated a claim of responsibility for the bomb attacks, in the name of the extremist group's Aamaq news agency.
"Attacks by Islamic State fighters hit gatherings of Alawites in the cities of Tartus and Jableh on the Syrian coast," the statement read.
The claim could not be independently verified by dpa, but it was consistent with previous statements by the extremist group, which controls large areas of inland Syria and views the Alawites as heretics.
Al-Assad's ally, Russian President Vladimir Putin, sent the Syrian leader his condolences in a telegram condemning what he termed "a bloody war against the Syrian people," Russian state news agency TASS reported.
Both cities are closely linked to the Russian involvement in the Syria conflict. Hmeimim, just outside Jableh, is the headquarters of the Russian forces, which launched a mainly aerial intervention in support of al-Assad's overstretched forces last year.
Tartus hosts Russia's only naval facility in the Mediterranean and has been key to resupplying Moscow's expeditionary forces in Syria.
The Syrian Foreign Ministry accused Saudi Arabia, Turkey and Qatar, which back rebel groups, of being behind Monday's bombings.
The opposition Syrian National Coalition denounced in a statement both attacks, but blamed the violence across the country on the Syrian regime.
"The National Coalition confirms the responsibility of the Al-Assad regime, directly or indirectly, for all terrorist operations targeting civilians in Syria," the statement said.