Suicide attacks claimed by the Islamic State extremist group killed 53 people in Syria on Monday, as US President Barack Obama said talks with his Russian counterpart Vladimir Putin had not yet "closed the gap" on a ceasefire agreement for the country.
The worst of Monday's attacks killed 38 people just outside the coastal city of Tartus, a government stronghold, the Syrian Observatory for Human Rights reported.
At least 16 of those killed were troops and pro-government militiamen, the Observatory said.
Tartus, much of whose original population belongs to President Bashar al-Assad's Alawite Muslim sect, also hosts large numbers of refugees from elsewhere in Syria.
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, the observatory said.
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.
Islamic State claimed all the attacks via its Aamaq media outlet.
Homs, Tartous, al-Hassakeh and government-held suburbs of Damascus have all seen repeated suicide attacks during the conflict, some on civilian targets and some on regime forces.
Many of the attacks, which have caused hundreds of casualties, have been claimed by Islamic State.
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."