Mustafa Bali, the head of the SDF media office, described the assault as the "last battle".
The besieged IS enclave centred around the town of Baghouz on the eastern bank of the Euphrates River represents the militant group's last territorial foothold in the part of Syria where the US-backed forces have been fighting it.
Kurdish-led forces spearheading the offensive have paused operations but US President Donald Trump predicted Wednesday that a final declaration of victory would come next week.
It said the US -led coalition struck positions of the IS in the eastern Euphrates to practice more pressure on the group.
That would herald an end to the almost five-year-old war aimed at pushing the Islamic State out of its self proclaimed "caliphate", the once vast stretch of territory spanning Syria and Iraq that at its peak was roughly the size to Britain.
America's allies on the ground announced the start of what they're calling the final battle against remaining ISIS holdouts after allowing thousands of civilians to flee.
US and SDF officials say, however, they have no reason to believe he is there.
Islamic State leader Abu Bakr al-Baghdadi was reportedly attacked in a village near Hajin by some of the terrorist organisation's foreign fighters in an apparent coup attempt, The Guardian reported, citing anonymous intelligence sources.
On Thursday, the coalition said the "caliphate" had massively shrunk.
Turfing out ISIS from the final 1%, comprising two villages, Marashida and Baghuz Fawqani, near the Iraqi border, will be the focus on the SDF operation tonight.
The SDF, which includes the Kurdish People's Protection Units (YPG), backed by Americans, has been a bone of contention in Washington's ties with Ankara. There are some ISIS fighters, CNN heard from SDF commanders, who want to surrender and others who want to fight until the death.
The country's Kurds have largely stayed out of the civil war, instead of developing semi-autonomous institutions in northern and northeastern Syria.
The U.S. -led coalition has said it is not discussing the timeline of the U.S. withdrawal from Syria.