At least 148 people have been killed in multiple attacks claimed by the Islamic State of Iraq and the Levant group in areas controlled by the Syrian government, a monitoring group said.
Syrian state TV also reported the attacks, putting the death toll at 78.
Simultaneous car bombs and suicide bombers hit bus stations, hospitals and elsewhere in the coastal cities of Tartus and Jableh on Monday, appearing to severely breach government defences, according to the Britain-based Syrian Observatory for Human Rights.
The attacks were the first of their kind in Tartus, capital of Tartous province and home to a Russian naval facility, and in Jableh in Latakia province, near a Russian-operated air base.
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.
The Observatory said 148 people were killed in series of attacks. It said there were seven explosions that ripped through both locations simultaneously: Four in Jableh, including three suicide bombs and one car bomb, and four in Tartus, including two suicide bombers and one car bomb.
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US State Department as well as UN Secretary General Ban Ki-moon condemned the attacks.
The Kremlin said the bomb blasts underlined the need to press ahead with the Geneva peace talks after the collapse of a February 27 ceasefire owing to intensifying violence in a war that has killed at least 250,000 people.
Russian President Vladimir Putin reiterated his readiness to fight with the Syrian government against "the terrorist threat" and sent his condolences to Assad, the Kremlin said.
Syria's Information Minister Omran al-Zoubi said in an interview with Ikhbariya that terrorists were resorting to bomb attacks against civilians instead of fighting on the frontlines, and vowed to keep battling them. The government refers to all rebels fighting against it as terrorists.
Hospital blast
In Jableh, dozens were killed when a car bomb went off near a bus station, followed by a suicide bomber who detonated his explosive belt inside the station. Two men blew themselves up at the electricity company and outside the emergency entrance of a city hospital.
Dozens more were killed in Tartus when a car bomb went off in the bus station, and then two men blew themselves up when people gathered, according to the observatory.
ISIL claimed responsibility for the attack through one of its media arms, Amaq.
"It is the first time in this war that simultaneous attacks of this scale took place in Latakia," Al Jazeera's Stefanie Dekker, reporting from Gaziantep in neighbouring Turkey, said.
