Militants of the Islamic State group killed at least 135 people and abducted 400 civilians during a daylong assault in the eastern Syrian city of Deir el-Zour, the Syrian Observatory for Human Rights said Sunday. According to the U.K.-based monitoring group, at least 85 civilians and 50 pro-government forces were killed in Saturday’s attack, which also saw the Sunni militant group aka ISIS make significant advances in the region.

“After their attack on Deir el-Zour, ISIS abducted at least 400 civilians from among the residents of the Al-Baghaliyeh neighborhood it captured, and adjacent areas in the northwest of the city,” the monitoring group said, citing “several reliable sources.”

“Those abducted, all of whom are Sunnis, include women, children and family members of pro-regime fighters,” Observatory chief Rami Abdel Rahman reportedly said.

Earlier after the monitoring group published a report saying that 135 people had been killed by the militant group in the city, Syria’s state news agency SANA said “around 300 civilians” — most of them women, children and elderly people — were killed in the “appalling massacre.”

The casualties reportedly took place in the Al-Baghaliyeh neighborhood, located in parts of Deir el-Zour province under ISIS control. If the numbers are confirmed, it would mark one of the bloodiest days in Syria’s five-year conflict.

“The legal and moral responsibility for this barbaric and cowardly massacre committed by ISIS hordes lies on the shoulders of all the states that support terrorism and that fund and arm takfiri [Sunni extremist] organizations,” the SANA news agency quoted Syrian Prime Minister Wael Nader al-Halqi as saying.

This is not the first time ISIS, which still remains in control of large parts of Syria and Iraq, has been accused of having committed a large-scale slaughter. In 2014, after the group overran the northern Iraqi town of Sinjar, its members are believed to have killed, raped and enslaved thousands of Yazidis — who practice an ancient religion predating Islam and Christianity, and are considered “devil worshippers” by ISIS — in attacks that the United Nations said indicated an attempt to commit genocide.