Six explosions claimed by the Islamic State of Iraq and the Levant (ISIL) hit government-controlled areas and a province held by Kurdish YPG forces in Syria on Sept. 5, killing 53 and injuring dozens of people, state media and a monitor said.
The IS-run Aamaq news agency says the group was behind the "simultaneous" attacks on Monday.
The worst of Monday's attacks killed 38 people just outside the coastal city of Tartus, a government stronghold from Alawites, the Syrian Observatory for Human Rights reported.
At least 16 of those killed were troops and pro-government militiamen.
Shortly after the Tartus bombings, at least four people were killed when a car bomb exploded in Homs, central Syria, which is also held by the government.
The attack hit the city's al-Zahraa neighbourhood, also mainly populated by Alawites. Footage of the aftermath on state television showed rubble covering the streets and smoke rising from burning vehicles.
A third attack targeted a Syrian army checkpoint in the al-Sabura area west of the capital Damascus, killing three people.
Meanwhile, the observatory reported at least eight people killed in a blast targeting a checkpoint manned by Kurdish security forces in al-Hassakeh, the largest city in north-eastern Syria.
As the violence continued in Syria, signs of possible progress between the United States and Russia on a Syrian ceasefire deal came to nothing after talks between the countries' leaders and top diplomats stalled.
"Technical" disagreements remain, a White House official said Monday.
US President Barack Obama and Russian President Vladimir Putin met for 90 minutes for talks centred largely on efforts to reduce violence; allow humanitarian aid; and to target Islamist groups.
Obama said the US and Russia "haven't yet closed the gaps" on a ceasefire agreement, a day after Russian-backed government forces advanced against rebels and reimposed a siege on the rebel-held eastern sector of Aleppo, Syria's largest city.
Obama added the nations were seeking a "meaningful, serious, verifiable" ceasefire and ways to provide humanitarian relief.
The leaders told their top diplomats to continue trying to reach an agreement amid "technical" issues having to do with implementation of the agreement.
Kerry told reporters Sunday after talks with Lavrov that there were still "a couple of tough issues" to work out and that efforts would continue to "see whether or not it is possible to bridge the gap."
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