WASHINGTON: United States and Russia announced on Monday that a cessation of hostilities in war-torn Syria will go into effect on February 27, as fierce clashes raged in the country's north.

The announcement comes just one day after the deadliest attack in the nearly five-year war, with 134 people — mostly civilians — killed in a series of blasts near Damascus.

In a joint statement, Russia and the United States said the cessation of hostilities would not apply to the Islamic State group (IS) or Al-Qaeda affiliate Al-Nusra Front.

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The statement said other parties engaged in fighting on the ground must indicate to Russia or the US whether they would take part in the agreement by midday Damascus time on February 26.

It said the opposition and the Syrian government must agree to allow “rapid, unhindered, and sustained” humanitarian aid access and to cease all attacks — whether aerial bombardment or otherwise -- on other parties to the agreement.

UN Secretary General Ban Ki-moon welcomed the deal as a “long-awaited signal of hope”, and urged all sides to abide by it.

A spokesman for the High Negotiations Committee — the main grouping of opposition factions in Syria — said rebel groups were “studying the agreement”.

Riad Naasan Agha told AFP that the HNC had discussed the ceasefire with the US special envoy to Syria Michael Ratney on Monday, but would not comment on whether it will agree to the proposal.

Deadliest attack in war

US President Barack Obama and Russian counterpart Vladimir Putin are set to speak “in the next days or so” on the terms of implementing the agreement, according to US Secretary of State John Kerry.

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.

On Monday, the UN commission of inquiry said civilians remain the “primary victims” in Syria's war and that accountability for war crimes must be part of the peace process.

“Paradoxically, the international and regional stakeholders that are ostensibly pushing for a peaceful solution to the war are the same that continue to feed the military escalation,” the 31-page report said.

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The HNC, meeting in Riyadh on Monday, has said any ceasefire must include provisions for Russia, Iran and foreign militia forces that back the regime to stop fighting.

IS on Sunday claimed responsibility for two deadly attacks in regime-held areas, which a monitor said killed 134 people near the Sayyida Zeinab shrine south of Damascus 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.