At least 79 civilians have been killed within the last 48 hours as a result of ongoing airstrikes in Syria's Deir ez-Zour, a local activist said Friday.
He said that the Syrian army provided humanitarian corridors for the exit of civilians from the city.
The Observatory said Russian air strikes on Thursday night killed 14 people, including three children, fleeing across the Euphrates on rafts near Mayadeen.
With support from Syrian Air Force fighter jets, government troops pounded the positions of Daesh terrorists in the towns of Mayadin, al-Muhasan, al-Boleel, al-Hussainiyah, al-Junaynah, al-Tayyebah and Saker Island in addition to Sheikh Yassin, al-Jubailah and al-Rashidiyah neighborhoods.
Mayadeen has been under IS control since 2014, when the group swept across swathes of Iraq and Syria and proclaimed a "caliphate", but regime forces this week advanced to within five kilometres (three miles) of the town.
Russian-backed Syrian regime forces today broke into the eastern town of Mayadeen, one of the Islamic State group's last bastions in the country, a monitor said.
Daesh terrorists receive weapons, arms and manpower from the Iraqi territory via the border town of Abu Kamal, according to SANA, the Syrian state news agency.
Moscow has been carrying out relentless air strikes in support of its ally Damascus targeting both ISIL in Deir Ezzor province and rival jihadists led by Al Qaeda's former Syria affiliate in Idlib province in the northwest.
The city, further up the Euphrates, was the de facto Syrian capital of IS's now collapsing "state". The observatory said on Friday there were clashes in several areas in eastern Deir Ezzor province. On Tuesday, it said a US-led coalition strike in Raqqa killed at least 18 civilians.
Hundreds of civilians across Syria have been killed or seriously injured in a surge of airstrikes in the past week, the deadliest in the country since the battle for Aleppo a year ago.