A monitoring group says IS militants have kidnapped 400 people from the city
Islamic State fighters have kidnapped at least 400 civilians during an attack on a government-held city in Syria, say reports.
The hostages, including women and children, were taken as the extremist group captured new territory in Deir al-Zor.
The Syrian Observatory for Human Rights said those abducted were all Sunnis and included families of pro-government fighters.
They were taken to other areas under IS control in northwest Syria, it said.
"There is genuine fear for their lives, there is a fear that the group might execute them as it has done before in other areas," it added.
The EU's foreign policy chief says the latest atrocities in Syria make the urgent need for a political settlement even clearer.
Federica Mogherini says yesterday's deal with Iran shows what can be achieved through diplomacy.
The UN is set to launch negotiations in Geneva between all Syrian parties in the next few days.
Earlier, Syria's state-run news agency SANA said at least 300 people were killed in Saturday's "appalling massacre".
It said most of them were elderly people, women or children, while opposition activists said the majority were Syrian soldiers and pro-government militia.
The Syrian Observatory for Human Rights says it has received confirmation of at least 135 deaths - including government troops, Islamic State fighters and civilians.
Lebanon-based Al Mayadeen TV, which is close to the Syrian government, also reported a civilian massacre in the city.
It said IS killed dozens of people, including women and children, and threw their bodies in the Euphrates River.
The reports could not be independently confirmed.
If true, it would be one of the highest death tolls for a single day in Syria's four-year civil war.
Sporadic fighting is continuing between IS and Syrian President Bashar al Assad's forces in the northwest of Deir al-Zor.
The regime still controls around 40% of the provincial capital as well as a nearby military airport despite repeated attacks from IS.
Further north, airstrikes have killed 40 civilians, including eight children, in Islamic State's self-declared capital Raqqa on Sunday, according to the British-based Syrian Observatory for Human Rights.
It was unclear if the bombings were carried out by Syrian or Russian jets, the monitoring group said.