Two massive bomb attacks killed 88 people in government-held areas of Syria, according to a monitoring group, even as US Secretary of State John Kerry said a truce in the civil war could come into effect within days.

Two near-simultaneous attacks on Sunday with explosive-rigged cars killed 57 people in the central city of Homs, while at least 31 people were killed in a series of blasts in the Damascus suburb of Sayyida Zeinab, the Syrian Observatory for Human Rights said.

Both areas are controlled by government forces and have seen previous deadly bomb attacks, claimed by the Islamic State extremist group, in recent months.

The explosions in Homs hit the city's al-Zahraa district, most of whose residents belong to President Bashar al-Assad's Alawite sect. At least 39 of the dead were civilians, the Observatory said, with the identity of the others unclear.

Footage from state news agency SANA showed burned-out vehicles and damaged buildings around a crater in the street where one of the blasts took place. SANA put the death toll at 32.

A statement in the name of Islamic State, posted on social media, claimed responsibility for the attack. dpa was not able to authenticate the claim but it was consistent with previous statements by the group.

The Observatory said that at least one car bomb and two suicide attackers wearing explosive belts were involved in the attacks in Sayyida Zeinab, a largely Shiite southern suburb of Damascus.

The area, a government stronghold, is home to an important Shiite shrine. Pro-Iranian militias who have come to the aid of al-Assad's overstretched forces have boasted that they are defending the shrine.

The bombings come exactly a month after previous attacks that killed 71 people in Sayyida Zeinab, including 29 civilians and 42 pro-regime militiamen, according to figures published by the Observatory at the time.

Earlier on Sunday, Kerry said he had reached a "provisional agreement in principle" with his Russian counterpart Sergei Lavrov on terms for a truce - but there was no confirmation from Moscow.

Kerry, speaking in the Jordanian capital, Amman, said he had spoken with Lavrov an hour earlier and that the truce "could begin in the coming days," although he warned that the deal was "not yet done."

"I anticipate that our presidents ... may well speak somewhere in the next days or so in order to try to complete this task," Kerry said.

The US top diplomat's comments came two days after the temporary cessation of hostilities, agreed to by world powers meeting in Munich a week earlier, was originally due to go into effect.

The Russian Foreign Ministry confirmed, in a statement carried by the official TASS news agency, that Kerry and Lavrov had spoken about the "modality and terms for truce in Syria, with the exception for the operation against terrorists."

But Moscow, whose air campaign in support of al-Assad has enabled the Syrian leader to claim major victories on the ground in recent weeks, gave no indication that a deal was close at hand.

The head of the Syrian opposition's peace talks committee on Saturday said that rebel groups had agreed in principle to a temporary truce, on condition that it covered all front lines and was accompanied by humanitarian measures and the release of prisoners.

The Syrian president, in an interview with the Spanish newspaper El Pais, hinted that the proviso about battling terrorists could be yet another stumbling block to any truce actually taking effect.

Al-Assad said that "of course" there would be further fighting against terrorists, adding that Syria and Russia had "announced four names: Ahrar al-Sham and Jaish al-Islam and al-Nusra and ISIS [Islamic State]."

The US agrees that Islamic State and al-Nusra Front - the Syrian branch of al-Qaeda - are terrorist groups that will not be covered by any truce.

That in itself raises questions about how the truce would work, since al-Nusra fights alongside other rebel groups in most of Syria.

But the other two groups mentioned by al-Assad are among the largest rebel forces fighting on the ground in Syria and are seen by Western countries as essential to any peace process.

Syria's war, which started in 2011 after al-Assad's forces launched a brutal crackdown against protests, has cost a quarter of a million lives and driven half the country's populations from their homes, according to UN estimates.

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