Black smoke clouds rose over ISIL's final territory in Deir Az Zor province on Monday as coalition fighter jets fired missiles in support of the Kurdish-led militia known as the Syrian Democratic Forces (SDF).
More than 20,000 civilians, some of whom were the families of ISIS fighters, have been evacuated from the area in recent days, according to the SDF.
The SDF fought off an ISIS counterattack Sunday morning, said media officer Mustafa Bali, and were able to advance from the north and west.
An SDF spokesman later said that the battle was "very fierce" as the "most experienced" militants were defending their last stronghold.
As the operation enters its final stages, fleeing ISIS militants have been caught attempting to sneak past SDF checkpoints by any means possible.
France's Defence Minister Florence Parly visited the Iraqi side of the border on Saturday where France has a forward artillery base near al-Qaim, just a few kilometres from Baghouz. We want to keep our journalism open and accessible and be able to keep providing you with news and analyses from the frontlines of Israel, the Middle East and the Jewish World.
It should be noted that Trump's latest rhetoric seems a preparation for quick pullout, or big coming announcement: "It should be formally announced sometime, probably next week, that we will have 100% of the caliphate", the president said Wednesday during an anti-ISIL coalition speech at the State Department. But holed up in tunnels, trenches, and houses around Baghouz are an estimated 600 diehard ISIS fighters.
But the expected United States pullout has seen them grappling to mend ties with the Damascus regime, which is also against Kurdish self-rule. The US is already taking out heavy equipment from Syria and the withdrawal of soldiers could begin in weeks.
More than four years after the extremists declared a "caliphate" across large parts of Syria and neighbouring Iraq, several offensives have whittled that proto-state down to a tiny holdout.
Bali said there were "dozens of SDF hostages" captured by ISIL.
In recent weeks, USA officials have said IS has lost 99.5 percent of its territory and is holding on to under 5 square kilometers (under 2 square miles), where most of the fighters are concentrated in Syria.
Meanwhile, thousands of civilians are trapped in eastern Syria. But activists and residents say IS still has sleeper cells in Syria and Iraq, and is laying the groundwork for an insurgency. Voronkov also said the central leadership of ISIS maintains an "intent to generate internationally-directed attacks".
"In connection with [US prospective withdrawal from Syria], we hope that Damascus will manage to find an acceptable solution on the issue of restoring Syria's sovereignty over the eastern bank of the Euphrates during contacts with the Kurds and take into consideration interests of all local ethnic groups", the diplomat said.