The US and Russia say they agree on a potential military and intelligence cooperation deal in Syria.
Opposition activists in Syria say at least 66 people have been killed in fighting across the country in the past 24 hours.
They say Syrian government and Russian air strikes have carried out fresh bombardment of Aleppo and neighbouring Idlib province.
There were 26 air strikes in Idlib on Sunday alone, killing at least nine people, according to the activists.
“Perhaps nothing indicates more effectively how intense the fighting is in Syria than the daily death toll,” said Al Jazeera’s Reza Sayah, reporting from Gaziantep on the Turkey-Syria border.
“Rarely have we seen a day where scores of people have not been killed.”
Late on Sunday, at least 35 people were killed and many more injured, some critically, after a suicide bomber detonated his explosive vest inside a bus in Syria’s Idlib province, according to the Syrian Observatory for Human Rights.
The Syrian Observatory said the bus was carrying opposition fighters when the blast happened near the Atmeh border crossing.
Turkey’s CNN Turk television citing local sources reported that the explosion occurred at the entrance of Atmeh refugee camp in Idlib.
Turkey sent several ambulances to the scene of the attack. Eight injured rebels who were in critical condition were taken to hospitals in Turkey’s Hatay province. The rest are being treated in Syrian hospitals, Turkish media reported.
Authorities in Hatay said in an official statement on Monday that four of the injured fighters later died in Turkish hospitals.
Idlib is a major bastion of the Turkey-backed opposition to President Bashar al-Assad’s rule.
The Syrian Observatory had earlier reported 26 air strikes on Sunday across the province.
Rami Abdurrahman, the observatory’s head, said Russian and government air strikes have intensified since rebels launched a campaign on July 31 from Idlib to break a government siege of Aleppo’s opposition districts.
The raids have killed scores of civilians, he said.
For its part, the Local Coordination Committees, a grassroots network of activists, says that more than 180 civilians have been killed across Syria since Friday, including 22 children and 23 women.
At least 90 people were killed on Friday, while a further 83 civilian deaths were recorded by Saturday night, with most of them occurring in Aleppo province.
“Even with the deadly standards of this war, these death tolls are staggering,” Al Jazeera’s Reza Sayah, reporting from Gaziantep on the Turkish side of the Syria-Turkey border, said.
The Syrian Observatory also released figures on Sunday detailing the killing of at least 327 civilians, including 76 children and 41 women, in Aleppo over the last 15 days.
More than 100 of the 327 were reportedly killed in bombing by government warplanes, while 126 were killed in attacks by opposition fighters on government-held areas in Aleppo.
In the suburbs of the city, another 94 were killed by Syrian government bombardment, according to the observatory.