A victim of the bombing in Sayyida Zeinab, a predominantly Shiite Muslim suburb of the Syrian capital Damascus, arrives at a hospital. (SANA/via AP)

The Islamic State asserted responsibility for bombings on Sunday that killed dozens of people in two Syrian government strongholds, casting a shadow over intensified diplomatic efforts to broker a cease-fire to the civil war.

At least three explosions struck a suburb south of the Syrian capital, Damascus, leaving at least 50 people dead near the Sayyida Zeinab shrine that is revered by Shiite Muslims, according to Syrian state television. The area is a high-profile target for extremist Sunni groups such as the Islamic State, which sees Shiites as ­apostates.

A bombing earlier in the day also targeted the city of Homs, killing at least 34 people, according to the area’s governor, Talal al-Barazi. State-run media cited him as saying that two vehicles packed with explosives detonated inside the city, which has been struck by a recent string of militant attacks. A Russian-backed military offensive that began in September has helped Syrian President Bashar al-Assad’s forces solidify full control over the city.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, put the death toll near Damascus at more than 62 and as high 46 in Homs.

Media affiliated with the Islamic State said the group carried out both attacks. Last month, the group claimed responsibility for another bombing near Sayyida Zeinab that killed as many as 71 people.

Images posted on social media purported to show the aftermath of the two attacks, including charred vehicles and columns of smoke rising from scorched ­rubble.

The Islamic State appears to be shifting its tactics and targeting areas controlled by Assad’s government, raising speculation that it is responding to the loss of ground in both Syria and Iraq to U.S.-backed Arab and Kurdish ­opponents.

The group also claimed responsibility for a suicide attack that killed more than 50 Kurdish fighters in eastern Syria on Sunday, according to the SITE Intelligence Group. Those fighters were linked to the Kurdistan Workers’ Party, a Turkey-based separatist group that has helped fellow Kurds exploit unrest to carve out an autonomous enclave in northern and eastern Syria.

The Islamic State has tried to capitalize on Syria’s unrest by focusing on seizing territory from other non-state militants, including fellow Islamist extremists and anti-Assad fighters. But the recent escalation of attacks against Assad’s forces, some analysts say, could be seen as a way for the group to boost recruitment of militants who are Syrian nationals. Scores of foreign fighters who joined the Islamic State have died in battle, with the group struggling to replenish their numbers, analysts say.

Syrians tend to be more interested in battling government forces than helping the Islamic State build its “caliphate,” which spans parts of eastern Syria and northern Iraq.

The bombings came as Secretary of State John F. Kerry announced apparent progress in diplomatic efforts to reach a truce to fighting that has dramatically intensified because of Russia’s five months of military intervention in the conflict.

Kerry, speaking from the Jordanian capital, Amman, announced that a “provisional” agreement to impose a reduction in hostilities had been reached with his Russian counterparts. Efforts to broker such a cease-fire have faltered since a tentative agreement to impose a truce was reached by world powers in Munich on Feb. 12.