Bomb attacks in Syria's central city of Homs and near a shrine outside Damascus killed at least 87 people on Sunday, as Washington pursued efforts for 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.

Double car bombings killed at least 57 people and wounded dozens in the Al-Zahraa district of Homs on Sunday morning, the Britain-based Syrian Observatory for Human Rights monitor said.

Later at least 30 people were killed in a series of attacks, including a car bombing, near the Shia shrine of Sayyida Zeinab south of the capital, state television and the monitor said.

The Homs attack was the worst in the city since twin bombings hit a school in October 2014, killing at least 55 people including 49 children.

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 the Islamic State group 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.

IS claimed responsiblity for the attack, saying in an online statement that two its members had driven explosives-laden cars into crowds of residents.

The blasts outside Damascus saw a car bombing and two suicide attacks rip through the area of the Sayyida Zeinab shrine, killing 30 people and wounding dozens more, state television reported.

The Observatory gave a slightly higher death toll of 31 killed and said there were four attacks.

It was not immediately clear who was behind the attacks but at the end of January IS claimed bombings that killed at least 70 people near the shrine, which contains the grave of a granddaughter of the Prophet Mohammed and is revered by Shia Muslims.

- 'Provisional' ceasefire deal -

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 February 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, 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’.

- Turkey defends shelling Kurds -

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 IS 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.