At least 48 people have been killed and dozens wounded in five explosions across mostly government-controlled areas of Syria, according to state media reports.
Monday morning's blasts were later claimed by the Islamic State of Iraq and the Levant (ISIL) group.
The explosions hit the coastal city of Tartous, the central city of Homs and the suburbs of the capital Damascus, as well as the northeastern city of Hasaka, which is mostly controlled by Kurdish forces but where the government maintains a presence.
State media said at least 35 people were killed and 43 others wounded in a double bomb attack just outside Tartous, in the coastal province of the same name, which is a base of President Bashar al-Assad's government.
"Two terrorist blasts on Arzuna bridge, the first a car bomb and the second a suicide bomber who detonated his explosive belt when people gathered to help the wounded," Syrian state television said.
The Syrian Observatory for Human Rights (SOHR), a Britain-based monitoring group, put the death toll of the Tartous blasts at 38, adding that 14 of those killed were civilians including two women and one child.
The SOHR also said the blast hit a checkpoint belonging to the Kurdish Asayesh security forces.
In another attack in Hasaka, in the northeast of the country, state media reported eight people were killed and two wounded in an attack caused by an "explosives-packed motorcycle".
State media also reported a car bomb at Homs's Bab Tadmur roundabout at the entrance to the al-Zahra neighbourhood, which is under government control.
It said at least four people were killed and 10 wounded in the bombing, the latest in a series of attacks targeting al-Zahra.
State media also reported another bomb attack on the al-Sabboura road west of the capital Damascus, in which one person was killed, and three others wounded.
"The attacks seem to have taken place at the same time, which leaves many to wonder whether this was a coordinated attack," said Al Jazeera's Hashem Ahelbarra, reporting from Gazientep on the Turkish side of the border with Syria.
Renewed Aleppo siege
Monday's bombings came just hours after diplomats from the US and Russia once again failed to reach a deal to ease the fighting.
A senior US State Department official said fresh crisis talks between John Kerry, the US secretary of state, and his Russian counterpart Sergey Lavrov on the margins of the G20 summit in China had ended without agreement.
A deal to provide aid to Aleppo's civilians and at least partially halt Russian and Syrian bombardments had looked possible on Sunday, before the talks collapsed.
US officials accused Russia of backtracking on already agreed issues which they refused to revisit, but the talks seemed to have been overtaken by developments on the ground.
Syrian government troops renewed their siege of Aleppo on Sunday.
"The blasts happened almost a day after the Syrian government re-took Ramosa which is a vast military complex taken over by the rebels last month," Al Jazeera's Ahelbarra said.
State media said they had taken an area south of the city, severing the last opposition-held route into Aleppo's eastern neighbourhoods.
Once Syria's economic powerhouse, Aleppo has been devastated by the civil war that began with protests against Assad's government in March 2011.