Nearly two dozen Turkey-backed rebel fighters killed in the bomb blast at the Atmeh crossing, bordering Idlib province.
At least 20 people, most of them rebels, were killed in a car bomb attack on Thursday that targeted an opposition checkpoint in northern Syria.
The blast – which hit a checkpoint run by a Free Syrian Army (FSA) affiliate near the city of Azaz, near the border with Turkey – sent dozens of wounded to nearby hospitals.
At least 14 of the dead were believed to be rebel fighters, according to the UK-based Syrian Observatory for Human Rights.
The death toll was likely to rise because of the number of seriously injured, according to the Observatory, which uses a network of sources on the ground to monitor the five-year-old conflict.
The checkpoint was operated by the Levant Front, a group connected to the FSA that is active in Aleppo province. It was on the road to the Bab al-Salamah crossing into Turkey.
The area is a stronghold of Turkish-backed Syrian rebels involved in a major operation aimed at clearing Islamic State of Iraq and the Levant (ISIL, also known as ISIS) from the border region.
ISIL has regularly targeted rebel factions with bombings, including an October 6 attack at a border crossing in neighbouring Idlib province which killed 29 opposition fighters.
Elsewhere, air strikes continued to pound rebel-held neighbourhoods of Aleppo on Thursday, where as many as 100 people have been killed by Russian and Syrian government missiles over the past three days.
As world powers prepared for new truce talks at the weekend, Russia announced it was ready to give rebels safe passage out of the eastern sector of Aleppo, where more than 275,000 people are under a government siege.
“We are ready to ensure the safe withdrawal of armed rebels, the unimpeded passage of civilians to and from eastern Aleppo, as well as the delivery of humanitarian aid there,” Russian Lieutenant-General Sergei Rudskoy said in a televised briefing.
Early morning raids in east Aleppo killed at least seven civilians, the Observatory said, and government forces captured high ground overlooking opposition areas on the northeastern outskirts of the city.
The Observatory said five children were killed by rebel rocket fire on western government-held neighbourhoods, with state television saying a school had been hit.
Residents in the west said they had been forced to pull over in their cars to take shelter in buildings because of the barrage of rebel fire.
Moscow has come under mounting international pressure over the rising civilian death toll from President Bashar al-Assad’s Russian-backed campaign to take east Aleppo, including Western accusations of possible war crimes.
Analysts said Thursday’s safe passage offer was simply a gambit to relieve the pressure by appearing to present diplomatic alternatives.
“There is no change in the Russian strategy: the goal remains the destruction of rebel presence in Aleppo,” Syria analyst Thomas Pierret told AFP news agency.
“Blowing hot and cold allows them to reduce the pressure and empower those who want a strictly diplomatic approach to the Syrian question.”
Since the army’s assault began in late September, Russian and government bombardment has killed more than 370 people, including 68 children, according to an Observatory toll.
Shelling by rebel groups, meanwhile, has killed 68 people in government-held areas.
Several major international efforts have failed to secure a political solution to Syria’s brutal war, which has cost more than 400,000 lives since 2011.
As key powers geared up for a new round of talks, newly appointed UN chief Antonio Guterres said Thursday was time to overcome divisions over ending the war.
“Whatever divisions might exist, now it’s more important to unite,” Guterres said. “It is the moral obligation of us all to stop the suffering of the Syrian people.”
US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov are expected to be joined at talks in the Swiss city of Lausanne on Saturday by their counterparts from Turkey, Saudi Arabia and Qatar – all backers of Syrian opposition forces.
Then in London on Sunday, Kerry is likely to meet his European counterparts from Britain, France and Germany.
UN Syria envoy Staffan de Mistura will also attend the Lausanne talks.