SYRIA (AFP)- A string of suicide bombings near a Shiite shrine outside Syria's capital and in Homs claimed by jihadists killed at least 127 people on Sunday (Feb 21), as Washington and Moscow worked to secure a ceasefire.

US Secretary of State John Kerry said a provisional deal had been reached on the terms of a truce, as violence intensified on the ground.

Near Damascus, the attacks, including a car bombing, ripped through the area of the Shiite shrine of Sayyida Zeinab and killed 68 people, the Syrian Observatory for Human Rights said.

Two car bombs killed at least 59 people and wounded dozens in the pro-regime district of Al-Zahraa in the central city of Homs, said the Britain-based monitoring group.

The Islamic State in Iraq and Syria (ISIS) said it was behind the day's carnage, saying in an online statement that two its members had driven explosives-laden cars into crowds of residents.

Al-Zahraa - whose residents are mostly from the same Alawite sect as Syria's ruling clan - has been regularly targeted by attacks, including last month when a double bombing claimed by ISIS killed 22 people.

State television footage from the scene showed emergency workers carrying a charred body on a stretcher past shops shorn of their fronts and mangled cars and minibuses.

An AFP reporter said the blasts struck about 400 metres from the shrine which contains the grave of a granddaughter of the Prophet Mohammed and is revered by Shiites.

At least 60 shops were damaged and cars reduced to mangled metal in the area, where a January attack also claimed by ISIS killed 70 people.

World powers have been pushing for a halt in fighting in Syria that was meant to take effect by Friday, but have struggled to agree on how it should be implemented.

The latest bombings came as Kerry said he had spoken with Russian counterpart Sergei Lavrov and agreed on how to implement a ceasefire.

"We have reached a provisional agreement, in principle, on the terms of the cessation of hostilities that could begin in the coming days," Kerry said in Amman.

"It is not yet done and I anticipate that our presidents, President (Barack) Obama and President (Vladimir) Putin, may well speak somewhere in the next days or so in order to try to complete this task," he added.

World powers proposed the truce just over a week ago as part of a plan that also included expanded humanitarian access, in a bid to pave the way for the resumption of peace talks.

The talks, which collapsed earlier this month in Geneva, were scheduled to resume on Feb 25, but the UN's envoy on Syria has already acknowledged that date is no longer realistic.

On Saturday, a key opposition umbrella group, the High Negotiations Committee (HNC), said it would agree a temporary truce only if regime backers halted fire.

HNC chief Riad Hijab said any ceasefire must be reached "with international mediation and with guarantees obliging Russia, Iran and their sectarian militias and mercenaries to stop fighting".

Assad meanwhile told Spain's El Pais newspaper that he was "ready" for a ceasefire, but said it should not be exploited by "terrorists".

Regime backer Moscow is a key architect of the proposed ceasefire, but has shown little sign so far that it plans to rein in the air campaign it began in September in support of Assad's government.

On Saturday, the Kremlin said it would continue "to provide assistance and help to the armed forces of Syria in their offensive actions against terrorists".

Regime forces backed by Russian strikes were advancing on Sunday east of Aleppo city against IS, consolidating their control over a stretch of highway from the city to the Kweyris military base.

The Observatory said at least 50 ISIS fighters had been killed in clashes and Russian strikes in the advance since it began Saturday morning.

Tensions meanwhile have been rising between Russia and opposition-backer Turkey, which has been alarmed by both the regime's Russian-backed advances and a major operation by Kurdish-led forces in Aleppo province.

The Kurdish People's Protection Units (YPG) and their Arab partners have in recent days seized key territory from rebel forces in Aleppo, prompting Turkish anger and shelling of their positions.

Ankara considers the YPG an affiliate of the outlawed Kurdistan Workers' Party, which has waged a decades-long insurgency against the Turkish state. It fears the Kurdish advances are intended to link up areas in north and northeast Syria to create a contiguous semi-autonomous Kurdish zone along the Syrian-Turkish border.

On Sunday, Turkish President Recep Tayyip Erdogan defended his country's fight against the YPG as "legitimate defence" after international calls for Ankara to halt its military action in Syria.