An estimated 148 people were killed Monday as multiple explosions struck 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, which dpa could not independently verify.
The attacks are the deadliest in decades in the coastal region, which is firmly under government control and hosts hundreds of thousands of people fleeing conflict in Syria's inland provinces.
Analyst Charles Lister of the US-based Middle East Institute warned that the attacks appeared aimed at increasing sectarian tension, similar to deadly attacks claimed by the Islamic State movement 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 [Islamic State] 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.
Shortly after Monday's bombings, locals and regime loyalists attacked a camp for displaced people in Tartus, the Observatory reported.
The Britain-based monitoring group said security forces arrested tens of refugees in the area from mainly Sunni inland provinces.
"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.
He voice concern about the safety of the refugees.
Monday's attacks began when a car was detonated in a bus and taxi station in Jableh, followed shortly by a suicide attacker blowing himself up, according to the observatory.
Two more suicide attackers 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.
"Deliberately targeting civilians is a war crime, and anyone involved in preparing, ordering, or carrying out such a crime should be held accountable," Nadim Houry, deputy Middle East director at Human Rights Watch (HRW) said.
"While it may be impossible to influence the criminal behavior of [the Islamic State organization], Syria's multiple warring sides and the international parties that support them can and should do more not only to condemn such [Islamic State] attacks but also to ensure that they stop targeting civilians."
Pictures aired by state television showed heavy damage in the areas where the blasts took place and blood-stained 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.
The claim could not be independently verified by dpa, but it was consistent with previous statements by the militant 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.
The US condemned the attacks but called on Russia to advance a political solution to focus attention on the fight against Islamic State forces. Secretary of State John Kerry called Russian Foreign Minister Sergei Lavrov to urge him to pressure the Syrian regime to stop airstrikes against the moderate opposition and civilians.
Tartus and Jableh are closely linked to the Russian involvement in the Syria conflict.
Hmeimim, just outside Jableh, is the headquarters of the Russianforces, which launched a mainly aerial intervention last year in support of al-Assad's overstretched forces. 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 both attacks but blamed the violence across the country on the Syrian regime.