"After their attack on Deir al-Zor (on Saturday), IS abducted at least 400 civilians from the residents of the Al-Baghaliyeh suburb it captured and adjacent areas in the northwest of the city," the Syrian Observatory for Human Rights said.
"Those abducted, all of whom are Sunnis, include women, children and family members of pro-regime fighters," Observatory chief Rami Abdel Rahman said.
They were transported to areas under IS control in the west of Deir al-Zor province and to the border with the Raqa province to the northwest.
The IS attack on Deir al-Zor on Saturday killed at least 85 civilians and 50 regime forces, the monitor said.
Syria's state news agency SANA, quoting residents, said "around 300 civilians" were killed in the onslaught and denounced a "massacre".
If confirmed it would be one of the highest tolls for a single day in Syria's nearly five-year war.
According to the Observatory, the advance put IS in control of around 60 percent of Deir al-Zor city, capital of the province of the same name in an oil-rich region bordering Iraq.
Sporadic fighting between IS and regime forces continued on Sunday in the northwest of the city, the monitor said.
Russian warplanes carried out airstrikes in support of regime fighters in Al-Baghaliyeh overnight, it said.
The regime still controls parts of the provincial capital and a nearby military airport despite repeated attacks from IS.
