Islamic State militants have killed 300 people in an "appalling massacre" in eastern Syria, according to the Syrian government.
A further 400 civilians were said to have been kidnapped by Isis, monitors said on Sunday, with the families of pro-government fighters were among those targeted.
Most of those killed in day-long attacks on Deir al-Zor were elderly people, women and children, the state-run Sana news agency said.
Syria’s government condemned the killings which it described as a “horrific massacre against the residents of Begayliya in Deir al-Zor”.
A source close to the government side said some of those killed had been beheaded but it was not possible to independently verify the reports.
If correct, the killings are some of the worst carried out by the extremist group, which controls large parts of Syria and Iraq.
Opposition activists also confirmed the killings with the Britain-based Syrian Observatory for Human Rights, which documents all sides of the Syria conflict through activists on the ground, saying at least 135 people were killed.
It said around 80 of them were soldiers and pro-government militiamen, while the rest were civilians.
The group added that many of the victims were shot dead or beheaded and it also expressed concern for those kidnapped.
“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,” said the Observatory’s head Rami Abdulrahamn.
Deir al-Zor is the main town in a province of the same name. The province links Islamic State’s de facto capital in the Syrian city of Raqqa with territory controlled by the militant group in neighbouring Iraq.
Islamic State has previously carried out mass killings following military assaults in Iraq and Syria, including the slaughter of 200 soldiers captured from the Tabqa airbase in Raqqa province, and hundreds of members of the al-Sheitat tribe in Deir al-Zor in 2014.
The group, in control of most of Deir al-Zor province, has laid siege since March on remaining government-held areas in the city of Deir al-Zor.
Residents are facing severe food shortages and sharply deteriorating conditions. Of those under siege in the city, 70 per cent are women and children, and many have been displaced from their homes elsewhere and are living in temporary shelters.
At least 200,000 civilians are believed to be under siege from Isis in Deir al-Zor.
The United Nations said on Saturday said residents there face a severe food shortage and sharply deteriorating conditions.
Western parts of the city have been under siege by Islamic State militants since last March, leaving inhabitants with no electricity for more than 10 months and a water supply for only three hours a week, the UN said in a report.
Deir al-Zor is the most populous of about 15 besieged areas in Syria, where about 450,000 people are trapped and cut off from aid by the government, Islamic State and other insurgent groups involved in the country’s civil war.