Dozens of people have died in an offensive by so-called Islamic State on government-held areas of Deir al-Zour in eastern Syria, local media say.

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.

Meanwhile, the UN warned of "sharply deteriorating conditions" in the besieged areas.

Around 200,000 people were in these areas, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.

DAWN ASSAULT

The IS assault on government-held areas in the city began at dawn on Saturday, Syria's Local Coordination Committees (LCC) said.

A car bomb explosion was followed by a ground attack, the 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 the Syrian observatory, IS now controls around 60% of the city.

More than 250,000 people have been killed and millions displaced after almost five years of conflict in Syria.

Blockades have been a feature of Syria's civil war. Up to 4.5 million people live in hard-to-reach areas, including nearly 400,000 people in 15 besieged locations who do not have access to life-saving aid.

TOWNS UNDER SIEGE

March 2015: Foah and Kefraya in Idlib province are besieged by rebel groups and the al-Qaeda-affiliated al-Nusra Front, with an estimated 12,500 trapped.

July 2015: Madaya, near Damascus, is besieged by government forces and their allies in Lebanon's Shia Islamist Hezbollah movement.

September 2015: The situation in Foah and Kefraya worsens after the fall of a nearby government air base, where helicopters had been able to land with food supplies. Reports emerge of people eating grass to survive.

October 2015: UN delivers one month's supply of food rations for 20,000 people in Madaya.

December 2015: Dozens of wounded civilians and fighters evacuated from Foah, Kefraya, Zabadani and Madaya. Reports begin to emerge of people starving in Madaya.

January 2016: UN says it has received credible reports of people dying of starvation in Madaya. Warns of "sharply deteriorating" conditions in parts of Deir al-Zor.

WHAT’S HAPPENING IN SYRIA?

More than 250,000 Syrians have lost their lives in almost five years of conflict, which began with anti-government protests before escalating into a brutal civil war. More than 11 million others have been forced from their homes as forces loyal to President Bashar al-Assad and rebels opposed to his rule battle each other - as well as jihadist militants from Islamic State.

WHY ARE CIVILIANS UNDER SIEGE??

All parties to the conflict are using siege warfare, encircling populated areas, preventing civilians from leaving and blocking humanitarian access in an attempt to force opponents to surrender. Shortages of food, water, medicine, electricity and fuel have led to malnutrition and deaths among vulnerable groups.

WHERE ARE THE SIEGES?

Government forces are besieging various locations in the eastern Ghouta area, outside Damascus, as well as the capital's western suburb of Darayya and the nearby mountain towns of Zabadani and Madaya. Rebel forces have encircled the villages of Foah and Kefraya in the northern province of Idlib, while IS militants are besieging government-held areas in the eastern city of Deir al-Zour.