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.
"With the support of the Russian aviation regime forces entered Mayadin and took control of several buildings in the West of the city", - quotes the edition the head of the London-based Syrian monitoring centre for human rights, Rami Abdel Rahman.
The Syrian military shelled al-Mayadin overnight and Syrian and Russian warplanes have conducted hundreds of strikes, it said.
The ministry also confirmed the death of three senior ISIS commanders including Omar al-Shishani as a result of an earlier Russian strike.
In June, two USA 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.
The Observatory said on Friday there were clashes in several areas in eastern Deir al-Zor province.
In eastern Syria, meanwhile, 15 civilians, including children, were killed when a missile slammed into a government-held neighborhood in the city of Deir el-Zour on Thursday evening. Moscow has said its Syrian allies were attacked by militants coming from areas near a deconfliction zone overseen by the U.S.
The ministry's statement followed a claim by IS-linked Aamaq news agency, which said the group had downed a Russian helicopter south of Shiekh Hilal village in Hama.
Russian Defense Ministry spokesman Maj.
Elsewhere near the Jordanian border, the Russian military said the U.S. is using the refugees living in the Rukban desert camp as human shields to protect its military base in the southeastern town of al-Tanf.
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.
Sheikh Muhammad Amlah, a leader of tribesmen fighters alongside the Syrian army, was cited by the pro-government al-Watan daily on Friday as saying that the army is advancing toward Mayadeen city as well as the area of Muhasan and other areas on the eastern bank of Euphrates River.