The Britain-based Syrian Observatory for Human Rights said Russian air strikes on Thursday night killed 14 people, including three children, fleeing across the Euphrates on rafts near Mayadeen.
The Russian state RIA Novosti news agency quoted a Syrian army general as saying that the Syrian forces have fought their way into Mayadeen.
In June, two U.S. intelligence officials told Reuters that they believed Islamic State had moved most of its diminished command structure and propaganda team to Mayadeen, southeast of its former capital of Raqqa.
Military sources say that the Tiger Forces are advancing under cover of heavy rocket and howitzer fire which is now targeting IS positions and rally points in the city center and in the eastern districts of Al-Mayadeen. "Instead of the New Syrian Army, mobile ISIL (former name of IS - TASS) groups, like a jack in the box, carry out sabotage and terrorist attacks against Syrian troops and civilians from there", he said. Both the Observatory and DeirEzzor 24 also reported that an airstrike hit the village of Mehkan, just south of Mayadeen, and said it killed several families.
Helped by the Russian military and Iran-backed militias last month, the Syrian army's advance to Deir Ezzor lifted a three-year-long siege imposed by Islamic State on a government-held enclave in the city. However, no information has been received about any USA operations against the Islamic State during the six months of its existence.
"The incident occurred while the Mi-28 was escorting another helicopter, a Mi-8, carrying personnel of the Russian Center for the Reconciliation of Conflicting Parties", the Defense Ministry said.
IS jihadists have controlled Mayadeen in the eastern province of Deir Ezzor since 2014. Gen. Igor Konashenkov said that in the last 24 hours, Russian Aerospace forces in Syria were focused on destroying Daesh reinforcements with a large number of foreign mercenaries arriving from Iraq to the region of Abu Kamal along the Syrian-Iraqi border.
The Russian accusations likely reflect rising tensions as US -backed Syrian forces and the Russian-backed Syrian army both of which are battling IS race for control of oil and gas-rich areas of eastern Syria.