The Syrian Observatory for Human Rights also confirmed the report, saying the government forces reached the western outskirts of Mayadeen, deemed as the capital of the IS stronghold in Deir al-Zour.
As a result of an air strike, more than 60 foreign mercenaries from the former Soviet Union, Tunisia, and Egypt were killed in south of Deir ez-Zor.
ISIS has retained control of the Iraq-Syria border near Mayadeen for three years, using the area to move its fighters and resources between the two countries, which militant leader Abu Bakr al-Baghdadi declared to be a caliphate for the world's Muslims in July 2014.
Around 120 ISIS militants and nearly 60 foreign mercenaries were killed following a series of Russian air strikes in Syria over the past 24 hours, the defence ministry in Moscow said on Saturday.
A picture taken during a press tour provided by the Russian Armed Forces on September 15, 2017 shows a Russian soldier standing guard in a central street in Syria's eastern province of Deir Ezzor.
The ministry noted that the armed forces destroyed a command post of terrorists near Meyadin city of Syria and killed up to 80 militants, including nine militants coming from the North Caucasus.More news: Hillary Clinton's Response to Las Vegas Shooting Calls for Gun Control
The Observatory said Russian air strikes on Thursday night killed 14 people, including three children, fleeing across the Euphrates on rafts near Mayadeen, AFP reported.
About 120 Islamic State (IS) militants and more than 60 foreign mercenaries in Syria were killed in airstrikes launched by Russia over the past 24 hours, the Russian Defense Ministry said Saturday.
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.
The area has become a "black hole", posing a threat to Syrian army's offensive against the IS in eastern Der el-Zour province, he added.
Backed by Russian air power, the army has continued its advance towards Mayadeen from Deir Ezzor along the Euphrates valley.
Russian President Vladimir Putin said he would continue to support Assad till Syria is "completely sanitised".