Islamic State suicide attackers killed at least 50 people in a triple car bomb attack on Thursday among a group of refugees in northeast Syria, a medical source in the Kurdish Red Crescent said.
A large number of people were also injured by the three car bombs set off by the attackers, the source said.
The attack took place at Abu Fas, near the border of Deir al-Zor and Hasaka provinces, said a war monitor, the Britain-based Syrian Observatory for Human Rights, which said earlier that at least 18 people had been killed.
The dead included refugees fleeing the fighting in Deir al-Zor as well as members of the Kurdish Asayish security force, the Observatory reported. Syrian state television said dozens had been killed in the attack.
The jihadist group has lost swathes of its territory in both Syria and Iraq this year and is falling back on the towns and villages of the Euphrates valley southeast of Deir al-Zor.
The US-backed Syrian Democratic Forces alliance of Kurdish and Arab militias is pressing it from the north and a rival offensive by the Syrian army, supported by allies including Iran and Russia, is attacking it from the west.
On Wednesday it said it carried out an attack in the capital Damascus, where three suicide bombers detonated their devices near a police headquarters, killing two people and wounding six.
Aid agencies have warned that the fighting in eastern Syria is the worst in the country this year and that air strikes have caused hundreds of civilian casualties.