Over the past three days Russia has launched 18 bombing raids in the region as well as the cruise missile strikes from submarines, the Russian Defence Ministry is claiming.
The ministry said the strikes had helped to support attack raids by Syrian troops, the RIA news agency reported.
Meanwhile the Syrian government, which is backed by Russia has regained complete control of the eastern city of Deir al-Zor on Friday, representing a huge blow to the jihadists as their last stronghold in Syria crumbles.
The latest military offensives have reduced the so-called ISIS caliphate to a pair of border towns at the Iraq-Syria frontier.
Deir al-Zor, on the west bank of the Euphrates River, is the largest and most important city in eastern Syria, and is the centre of the country’s oil production.
A military source said: ”The armed forces, in cooperation with allied forces, liberated the city of Deir al-Zor completely from the clutches of the Daesh terrorist organisation.”
The army, backed by Russian bombers, Iranian and Shi’ite militias, is advancing towards the last significant town held by ISIS in Syria, Albu Kamal, which is also located on the western bank of the Euphrates.
A rival offensive by Kurdish and Arab militias supported by a US-led coalition with air strikes and special forces, is pressing ISIS fighters on the eastern bank.
Iraqi forces on Friday said they had begun their own final offensive against ISIS on the Iraqi side of the jihadist territory that straddles the border between the two countries, entering the city of al-Qaim.
The terrorist group had for years besieged a government enclave in Deir al-Zor until an army advance relieved it in early September, starting a battle for jihadist-held parts of the city.
The army captured al-Hamidiya, Sheikh Yassin, al-Ardhi and al-Rashidia districts in recent attacks and the al-Hawiqa district was the last to be held by the jihadists, a military media unit run by the army’s ally Hezbollah reported.
A US-led international coalition has also been bombing ISIS and supporting ground allies on both sides of the frontier said the militant group now has a few thousand fighters left, mainly holed up at the border in Iraq’s al-Qaim and its sister town of Albu Kamal on the Syrian side.
US Army spokesman Colonel Ryan Dillon said: ”We do expect them now to try to flee, but we are cognisant of that and will do all we can to annihilate ISIS leaders.”
He estimated there were 1,500-2,500 fighters left in al-Qaim and 2,000-3,000 in Albu Kamal.
But both the Iraqi and Syrian governments and their international backers say they worry that the fighters will still be able to mount guerrilla attacks once they no longer have territory to defend.
Mr Dillon said: ”As ISIS continues to be hunted into these smallest areas we see them fleeing into the desert and hiding there in an attempt to devolve back into an insurgent terrorist group.
“The idea of ISIS and the virtual caliphate, that will not be defeated in the near term. There is still going to be an ISIS threat.”
Driven this year from its two de facto capitals – Iraq’s Mosul and Syria’s Raqqa – Islamic State is pressed into an ever-shrinking pocket of desert straddling the frontier.