Syrian government troops meanwhile moved closer to the border town of Albu Kamal, the last urban zone still held by IS jihadist holdouts in Syria, the Syrian Observatory for Human Rights said.
The regime's forces "progressed rapidly" and are 15 kilometres (nine miles) from the town, the Britain-based monitor said.
Saturday's attack in the eastern province of Deir Ezzor killed "at least 75 displaced civilians including children" and wounded 140, said Observatory head Rami Abdel Rahman.
The scale of the attack showed that despite losing vast swathes of territory it held in Iraq and Syria, the jihadist group is still capable of mounting deadly attacks.
Abdel Rahman said "a new convoy of displaced people joined the gathering at the time of the attack".
The displaced had fled battles in the province, where Russian-backed Syrian regime forces and the US-backed Syrian Democratic Forces (SDF), are fighting the jihadist group in separate offensives.
The Observatory, which relies on a network of sources on the ground inside Syria for its information, reported Saturday that dozens had died in the bombing.
Fighting across Deir Ezzor province has sent thousands of civilians fleeing for their lives, some straight into the desert.
Saturday's attack came a day after Russian-backed government forces seized Deir Ezzor city from the jihadists.
IS, which in 2014 declared a "caliphate" spanning territory in Iraq and Syria roughly the size of Britain, has also lost most of the territory it once controlled in neighbouring Iraq, including second city Mosul.
- Fight for Albu Kamal -
On Friday, Iraqi forces retook the border town of Al-Qaim, also on the Euphrates river.
Iraqi Prime Minister Haider al-Abadi on Sunday toured Al-Qaim and raised the national flag in the town centre.
The jihadists who have lost most of the territory they once controlled are now confined to a small stretch of the Euphrates Valley border area, which was at the heart of IS's so-called caliphate.
The US-led coalition has estimated that around 1,500 jihadists are still in the area.
Syrian regime forces have intensified their assault on IS in the area in a bid to seize Albu Kamal close to the Iraq border.
According to the Observatory, Iraqi militiamen have fought alongside Iraqi regime forces in the area.
Civilians caught in the fighting have tried to escape.
- Hundreds of thousands fleeing -
Aid group Save the Children estimates that some 350,000 people have fled the recent fighting in the oil-rich province, half of them children.
Some of those displaced had sought refuge in a desert area controlled by the SDF, a Kurdish-Arab alliance, on the eastern bank of the Euphrates where Saturday's bombing took place, the Observatory said.
It was not the first attack attributed to IS and targeting civilians fleeing Deir Ezzor.
On October 12, a car bombing in the northeastern province of Hasakeh killed at least 18 people, including displaced people and Kurdish security forces, the Observatory said.
The jihadists have been repeatedly accused of targeting those attempting to flee during fighting.
The United Nations on Thursday said the group had executed 741 civilians in the battle for the Iraqi city of Mosul, accusing it of "indiscriminate targeting of civilians trying to flee the city".
Despite being driven out of large parts of Deir Ezzor, IS still controls 37 percent of the province and its fighters are deployed on the eastern side.