At least 29 poeple, mostly Syrian rebels, were killed Thursday in an explosion at Syria's rebel-controlled Atmeh border crossing with Turkey, a monitoring group reported.
The Syrian Observatory for Human Rights said that the explosion occurred when a man detonated his explosive belt among a crowd of Turkish-backed Syrian opposition fighters.
Atmeh, some 40 kilometres west of Aleppo, is a vital crossing point used for delivering aid and supplies to rebel-held areas in the north of Syria.
The Islamic State extremist group claimed responsibility for the attack.
Amaaq News Agency, the militants' mouthpiece, reported that the attack had been carried by a suicide car bombing that targeted opposition rebels at the crossing.
According to Amaaq, around 80 people were killed or injured in what it called a "martyrdom operation," a term used by jihadists to refer to suicide attacks.
In August, a suicide bombing at the same crossing, claimed by Islamic State, killed some 32 Syrian rebels.
Turkey invaded northern Syria in August to attack Islamic State and Kurdish militants, though in recent weeks it has turned its focus on the jihadist group after the United States urged Ankara, its NATO ally, to refrain from further attacks on the Kurds.
Rebels from Jaish al-Fath, a coalition dominated by hardline Islamist group Ahrar al-Sham and former al-Qaeda affiliate Jabhat Fatah al-Sham, control most of the territory on the Syrian side of the Atmeh border.
In recent weeks, Syria has seen a surge in fighting between government forces, backed by a Russian air power, and opposition fighters.
Russia has sent three missile-carrying warships to join its naval task force in the eastern Mediterranean in recent days, naval spokesman Nikolai Voznesensky said in comments carried by state media Thursday.
Syrian President Bashar al-Assad, supported by his Russian allies, launched a major offensive last month to dislodge rebels from the divided city of Aleppo in northern Syria.
Regime forces on Thursday advanced against rebels in the district of Bustan al-Basha in the northern sector of opposition-controlled eastern Aleppo, the Observatory said.
"With this advance, the regime now controls half of Bustan al-Basha," the watchdog's head, Rami Abdel-Rahman, told dpa.
"It is a major advance for the regime since the rebels took the area in 2013."
Syrian state television meanwhile said that government forces had recaptured key buildings in the area and pushed back "terrorists," a reference to rebels fighting to oust al-Assad.
Some 250,000 to 300,000 people are thought to remain in eastern Aleppo, which has been hit by devastating airstrikes and artillery shelling, including the repeated bombardment of its few remaining hospitals in recent weeks.