Islamic state militants have reportedly killed hundreds of people execution style in attacks on government-held areas in Syria's city of Deir al-Zor on Saturday (local time), a source and a monitoring agency said.
The Syrian Observatory for Human Rights said 85 civilians were killed in the attack, as well as 50 Syrian soldiers.
Syria's state news agency Sana reported that around 300 civilians had died in the Baghaliyeh area of the city.
Some of the casualties were beheaded, a source close to the Syrian government said.
A Syrian source said the group "committed a massacre among the civilians."
"They sent six suicide bombers first and they tried to break into military positions but they failed."
A car bomb explosion was followed by a ground attack, Syria's Local Coordination Committees (LCC) said.
Government forces retaliated with air strikes and heavy artillery, the report said.
Russia said it had dropped aid to civilians caught up in the siege. There have also been reports of Russian airstrikes in the vicinity.
According to Syrian Observatory for Human Rights (SOHR), which monitors the violence in the country through a wide network of local sources, IS now controls around 60 percent of the city.
More than 250,000 people have been killed and millions displaced after almost five years of conflict in Syria.
Islamic State is in control of most of the eastern province while the government is holding parts of the city including a military airport.
Deir al-Zor province links Islamic State's de facto capital in Raqqa with territory controlled by the group in Iraq.
Government-held areas in the city had been under siege by Islamic State fighters for more than a year and more than 200,000 people there are living in dire conditions lacking food and medicine.
The Syrian source said the group has been trying to attack the city almost on a daily basis and on Saturday it carried out several assaults.
Islamic state supporters on social media said the group had also captured an army weapons depot and seized tanks.
However, these reports were unable to be independently verified.
SOHR also said unknown warplanes carried eight air strikes on the city of Raqqa killing at least 16 people and wounded 30 others.
-BBC / Reuters