At least 29 people have been killed in an explosion in a village in north-western Syria, near the border with Turkey.
The Islamic State group quickly said it was behind the attack in Atmeh, in which several Turkey-backed opposition fighters were killed.
The bombing underscored the complex layers of the Syrian conflict, where a civil war between Syrian President Bashar Assad's forces and the rebels trying to oust him is taking place alongside a militant insurgency and an international war against IS.
Meanwhile, activists in the northern city of Aleppo said it was calmer on Thursday after Syria's military command announced the night before it would scale back bombardment of the contested city to allow civilians to leave besieged rebel-held neighbourhoods.
"There were shellings and air raids, but it was less than in previous days," said activist Bahaa al-Halaby, speaking from Aleppo province near the city.
The Britain-based Syrian Observatory for Human Rights said that although air strikes have almost stopped on the eastern rebel-held areas, government forces are pushing ahead in their ground offensive.
The Observatory and state media said government forces advanced in Aleppo's northern neighbourhood of Bustan al-Basha, where troops captured a sports complex and a nearby housing compound.
In the explosion in Atmeh, activists said the blast occurred as opposition fighters gathered near a small restaurant in the area. It also happened during an exchange of forces going to Syria and those returning to Turkey, according to Rami Abdurrahman of the Observatory.
The Atmeh border post is one of several crossings Syrian rebels use to bring in fighters and supplies.
The Observatory said the blast was caused by a suicide bomber wearing an explosives belt, and many other people were injured.
Another group, the Local Co-ordination Committees, said the blast was caused by a bag filled with explosives that went off on the Syrian side of the border crossing, killing 35 people including two senior judicial officials from the opposition.
An amateur video posted online shows about 18 bodies lying on the ground outside what appears to be a clinic. Some of the men are in military uniforms.
In the Islamic State group's claim, the IS-linked Aamaq news agency said a suicide car bomber struck a convoy of rebels on the Syrian side of a border crossing point in Atmeh.
Turkish deputy PM Numan Kurtulmus described the blast as an "inhuman attack", adding that "those who perpetrated this committed a crime against humanity".
Turkish state-run Anadolu Agency said the attack occurred during a "change of guard" by the Turkish-backed forces at a refugee settlement.
Turkey sent tanks into Syria on August 24 as part of the so-called Euphrates Shield operation aimed at ousting Islamic State fighters and halting an advance by Syrian Kurdish forces, which Ankara sees as allies of Turkey's own outlawed Kurdish militia that has been waging a 30-year insurgency inside Turkey.