Efforts intensified for a partial truce in Syria as fighting raged near Aleppo on Monday and after the country suffered its bloodiest jihadist attack in nearly five years of war.
US President Barack Obama and Russian counterpart Vladimir Putin are expected to speak in the coming days after Washington announced a provisional agreement had been reached on an imminent "cessation of hostilities".
US Secretary of State John Kerry announced the deal on Sunday, as a string of suicide bombings in areas near a Shiite shrine outside Damascus and in the city of Homs killed at least 184 people.
The Islamic State group claimed responsiblity for both attacks in regime-held areas, which a monitor said killed 120 people near the shrine of Sayyida Zeinab and at least 64 in the Al-Zahraa district of Homs.
The bombings near the shrine marked the deadliest jihadist attack since Syria's conflict erupted in March 2011, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.
Kerry said the US and Russian leaders were to speak "in the next days or so" on the terms of implementing the agreement, which would apply to fighting between non-jihadist rebel forces and regime troops but not to IS and other extremists.
- Opposition talks in Riyadh -
Announced by top diplomats in Munich earlier this month, the ceasefire failed to take hold by last Friday as initially planned.
Part of a plan that also included expanded humanitarian access, the proposal aims to pave the way for a resumption of peace talks that collapsed earlier this month in Geneva.
The talks had been scheduled to resume on February 25, but the UN Syria envoy has acknowledged that date is no longer realistic.
Syria's main opposition umbrella group, the High Negotiations Committee (HNC), was meeting in the Saudi capital Riyadh on Monday for talks on the ceasefire and peace talks efforts.
Spokesman Monzer Makhous told AFP the meeting was expected to continue for two or three days.
The HNC has said any ceasefire must include provisions for Russia, Iran and foreign militia forces backing the regime to stop fighting.
- Key supply route cut -
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Russia launched air strikes in Syria last September against what it said were "terrorists" but has been accused of bombing non-jihadist rebel forces in support of President Bashar al-Assad, a longtime ally.
Iran has sent military advisors to Syria and the Tehran-backed Lebanese Shiite movement Hezbollah has deployed at least 6,000 militants to fight with Assad.
Iran would have to be on board for any ceasefire to work and Russian Defence Minister Sergei Shoigu made a surprise visit to Tehran on Sunday, saying he was delivering a "special message" from Putin to President Hassan Rouhani.
Efforts have been intensifying to resolve Syria's war -- which has left more than 260,000 dead, forced millions from their homes and devastated the country -- as the conflict approaches the five-year mark.
The rise of IS, which has seized large parts of Syria and neighbouring Iraq and become the preeminent global jihadist group, has focused attention on the need for a solution.
On Monday, top military commanders from 30 nations fighting IS as part of a US-led coalition gathered in Kuwait to discuss ways to defeat the jihadists.
"As military leaders, it is our responsibility to double efforts and urgently work out suitable plans to eliminate all terrorist groups threatening many countries in the world," said Kuwaiti chief of staff General Mohammad al-Khader.
IS's attacks on Sunday drew sharp criticism from Moscow, which said the blasts were aimed at undermining peace efforts.
"The atrocious crimes of extremists are aimed at scaring the peaceful population, subverting attempts to reach a long-term political settlement to the Syrian crisis," the Russian foreign ministry said in a statement.
Moscow's air strikes have helped regime forces make important gains, particularly in recent weeks around the second city of Aleppo.
But their efforts to take Aleppo city -- long divided into regime- and rebel-held areas -- were dealt a setback on Monday.
IS and other jihadists cut a vital supply route linking the west of Aleppo with other government-held territory, the Observatory said.
The road between Aleppo and the town of Khanasser to the southeast was the only way regime forces and civilians living in government-controlled neighbourhoods of the city could travel to surrounding provinces.
If government forces are unable to recapture the road, it could slow their offensive in the area.