Syrian activists and pro-government media say more than 140 people have been killed in a series of explosions in government-held areas of Syria.
The February 21 attacks came as U.S. Secretary of State John Kerry said a "provisional agreement" had been reached with Russia on a partial truce in the Syrian conflict.
The British-based Syrian Observatory for Human Rights monitoring group reported that at least 59 people, mainly civilians, were killed in a double car bombing in the central city of Homs.
The explosions hit the Al-Zahraa district, most of whose residents belong to President Bashar al-Assad's Alawite sect.
Homs city is almost completely controlled by the Syrian government.
Hours later, at least three blasts targeted a Shi’ite suburb of Damascus.
State television reported at least 87 dead and some 200 wounded. The observatory put the death toll at more than 80.
The attacks hit the Sayyida Zeinab district, which is home to the grave of a granddaughter of the Prophet Muhammad, one of Shi'ite Islam's holiest shrines.
The Islamic State (IS) group claimed responsibility for the Homs and Damascus blasts.
The observatory also said that at least 50 IS fighters had been killed in the past 24 hours in an advance by government troops, backed by Russian air strikes, east of the northern city of Aleppo.
Earlier, Kerry said he and his Russian counterpart, Sergei Lavrov, had reached a “provisional agreement” on terms of a cessation of hostilities in Syria.
"We have reached a provisional agreement in principle on the terms of a cessation of hostilities that could begin in the coming days," Kerry told a news conference in Amman with Jordanian Foreign Minister Nasser Judeh on February 21.
Kerry said he and Lavrov hoped that U.S. President Barack Obama and Russian President Vladimir Putin would be speaking in the coming days in order to complete the provisional agreement in principle.
He added that the two sides were "still filling out the details."
The Russian Foreign Ministry confirmed Lavrov and Kerry had spoken about conditions for a cease-fire in Syria on the telephone.
It said discussions were on cease-fire conditions, which would exclude operations against organizations "recognized as terrorists by the UN Security Council."
Meanwhile in Tehran, Russian Defense Minister Sergei Shoigu met with Iranian President Hassan Rohani.
The official IRNA news agency reported that Shoigu gave a message from Russian President Vladimir Putin to Rohani.
It also said the Russian minister presented an overview "of the situation regarding stability in the region and the process of negotiations for a cease-fire in Syria."
Russia and Iran have both been strong backers of President Assad during the nearly five-year conflict.
Major powers agreed in Munich on February 12 to work toward a “cessation of hostilities” in Syria and to expand delivery of humanitarian aid to people caught up in the conflict.
The Munich agreement between 18 countries supporting opposing sides in Syria's civil war had called for a truce to begin as early as February 19.