More than 140 people were killed in a series of bomb blasts Sunday in government-held areas of Homs and near the Syrian capital Damascus, as the top US diplomat said a truce in the civil war could come into effect within days.

At least 83 people were killed and more than 180 wounded in several blasts in Sayyida Zeinab, a largely Shiite southern suburb of Damascus, according to state news agency SANA.

The Syrian Observatory for Human Rights, a pro-opposition monitoring group, said two near-simultaneous attacks with explosive-rigged cars killed 59 people in the central city of Homs.

Statements in the name of the Islamic State movement, posted on social media, claimed responsibility for both attacks.

Dpa was not able to authenticate the claims, but they were consistent with previous statements by the group, including claims of responsibility for three other deadly attacks in the same areas since December.

The explosions hit Hom'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.

Video from SANA showed burned-out vehicles and damaged buildings around a crater in the street where one of the blasts took place. SANA put the toll at 39.

The Observatory said that at least one car bomb and two suicide attackers wearing explosive belts were involved in the Sayyida Zeinab attacks.

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.

US Secretary of State John Kerry said Sunday that he had reached a "provisional agreement in principle" with Russian Foreign Minister Sergei Lavrov on terms for a truce.

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

Moscow, whose air force has enabled Syrian government forces to make major gains in recent weeks, confirmed that the two sides had "agreed additional parameters for the ceasefire regime in Syria."

"The results of the conversations are being reported to presidents Vladimir Putin of Russia and Barack Obama of the United States," the Russian Foreign Ministry said in a statement reported by the official TASS news agency.

Kerry's comments came two days after the temporary cessation of hostilities, agreed to by world powers a week earlier in Munich, was originally due to take effect.

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 [the Islamic State group]."

The United States agrees that the Islamic State movement 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, because 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.

Russian Defence Minister Sergei Shoygu arrived Sunday in Tehran, meeting with Iranian President Hassan Rowhani to discuss the Syrian conflict and delivering a message from Russian President Vladimir Putin.

In a statement, Rowhani said Tehran and Moscow are in agreement that the Syrian conflict could only be resolved through diplomacy, and that Syrians themselves must decide the country's political future.

Rowhani's office gave no further details about the meeting or the communications from Putin. Both countries are close al-Assad allies.

The Syrian war, which started in 2011 after al-Assad's forces launched a brutal crackdown against protests, has cost 250,000 lives and driven half the country's population from their homes, according to UN estimates.

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