Syrians living in rebel-controlled areas fear a deal between Russia and the US has already been agreed.
Dozens of civilians have been killed in the Syrian city of Homs despite announcements suggesting that a temporary ceasefire is closer than ever.
Syria’s state news agency SANA said Sunday’s twin car bombing happened near the entrance to the city’s al-Arman neighbourhood.
The attack killed 46 people and wounded 110 more, Syrian officials said.
Homs is largely under government control and has regularly been targeted in bomb attacks.
The violence on the ground came on a day the US secretary of state said a “provisional agreement” had been reached on a ceasefire to end the Syrian conflict.
John Kerry, speaking in Amman alongside Nasser Judeh, Jordan’s foreign minister, said he had spoken earlier that morning with his Russian counterpart, Sergey Lavrov, about the agreement.
Now, he said, both the US and Russia planned to reach out to the various sides of the conflict.
Kerry said he hoped President Barack Obama and Russian President Vladimir Putin would talk soon and implementation could begin after that.
He said the details such as enforcement still needed to be resolved, and that the international community was “closer to a ceasefire today than we have been”.
The Russian foreign ministry confirmed that Lavrov and Kerry had spoken about conditions for a ceasefire in Syria on the telephone on Sunday.
It said discussions were on ceasefire conditions, which would exclude operations against organisations “recognised as terrorist by UN Security Council”.
On Saturday, a number of Syrian opposition groups declared that they agreed to the “possibility” of a temporary truce if President Bashar al-Assad’s government and its allies respect several conditions, including halting fire.
The groups said they would agree provided there were guarantees that government forces and its allies would respect a ceasefire, sieges were lifted and aid deliveries permitted across the country.
The declaration came as fighting continued on the ground despite a Friday deadline for cessation of hostilities.
The opposition factions “expressed agreement on the possibility of reaching a temporary truce deal, to be reached through international mediation”, a statement from the High Negotiations Committee said.
It said the UN must guarantee “holding Russia and Iran and sectarian militias … to a halt to fighting”.
All sides should cease fire simultaneously and the government should release prisoners, the statement said.
For his part, Assad said in an interview with Spanish newspaper El Pais on Saturday that he was ready to implement a ceasefire but only if the rebels and their international backers such as Turkey did not use it as a chance to gain ground.
The fighting in Syria started as an unarmed uprising against Assad in March 2011, but has since expanded into a full-on conflict that has killed more than 260,000 people, according to UN estimates.
Millions more have been displaced, having fled to neighbouring countries and Europe.