At least 22 people were killed and dozens injured in an explosion at a Kurdish wedding in the northeast Syrian city of Hasaka, a monitoring group said.
The Kurdish YPG, or People's Protection Units, said it was unclear whether the blast was caused by a explosive device or a suicide bomber.
But a statement by the ISIL-affiliated Amaq news agency said a suicide bomber had attacked a gathering of Kurdish YPG fighters on the edge of the city with machine guns and an explosive vest.
Hasaka is mainly in the hands of the YPG after it evicted the Syrian army in August.
ISIL also claimed responsibility for suicide attacks in the central Syrian city of Hama that killed three people and wounded 11 others earlier on Monday.
Two suicide bombers wearing explosive-laden belts carried out the attacks near the ruling Baath party office and a police station, Amaq reported.
Syria's state news agency SANA reported earlier that a suicide bomber detonated his explosives in al-Assi Square in Hama city, and the other blast occurred 15 minutes later.
Hama is Syria's fourth-largest city and has seen relatively little fighting in recent years as the country's conflict rages on. It is firmly under the control of President Bashar al-Assad's forces.
Hama province is of strategic importance to Assad as it separates opposition forces in rebel-controlled Idlib from Damascus to the south and the government-controlled coast to the west.
In 2013, a major push by rebel groups to capture Hama was repelled by government forces after reinforcements were sent to the area.
Major demonstrations erupted in Hama in 2011 as the war broke out, but they were quickly suppressed.
Assad's father and predecessor Hafez al-Assad brutally put down a Muslim Brotherhood uprising in Hama in 1982, killing thousands of people.
Elsewhere, at least 15 Turkish-backed Syrian rebels were killed in clashes with ISIL fighters as they moved on the group's strongholds in northern Syria, according to Turkish military officials.
About 35 Syrian rebels were wounded in the fighting, which seeks to capture seven residential districts south of the town of al-Rai.
According to a statement, "intense" clashes had taken place in the regions of Boztepe, Hardanah and Turkmen Bari and the casualties took place over the last 24 hours.
The bombardment of Aleppo by the Syrian government and its allies also intensified on Monday.
Government forces have taken territory to the north of the opposition-held enclave, and there's been fierce fighting along the front line that cuts through the divided city.
"It's not clear if the rebels, who have been besieged for weeks, can hold their ground," said Al Jazeera's Zeina Khodr, reporting from Gaziantep, on the Turkish side of the border.
"And it is not clear if the government will be able to win in what could be a long war involving street-to-street fighting."