The SDF launched an offensive to expel IS from the eastern province of Deir Ezzor in September.
ISIS said in a statement posted late Sunday that two of its "martyrdom-seekers" attacked SDF fighters in Baghouz with their explosive-laden vehicle.
Almost 600 ISIS fighters remain inside the two villages that comprise the enclave, and many of them are believed to be foreigners, he said.
There are an estimated 500 jihadists are holed up in the 3 square kilometre Baghouz area.
The fighting is expected to be over within days, Mr Bali said.
Votel was definitive that all USA troops will leave Syria, but he wouldn't put a public timeline on when the move would be completed, CNN reported.
While President Trump has put no firm deadline on withdrawing troops from Syria, he has directed the Pentagon to get all forces out of the country, an order that has led to the current USA military timeline to leave by the end of April.
While the SDF has warned that it is unable to detain growing numbers of ISIS fighters and their families long term, watchdog organisation Human Rights Watch has warned that the rendition of prisoners to Iraq to face trial risks breaching global and domestic laws.
Spokesman Bali said IS leader Abu Bakr al-Baghdadi, the man who pronounced the cross-border "caliphate" in 2014, was not among them, and likely not in Syria.
The strategy, which the group deployed in Mosul and elsewhere, is presenting a real challenge, SDF commanders said.
But the expected USA pullout has seen them grappling to mend ties with the Damascus regime, which is against Kurdish self-rule.
Meanwhile, thousands of civilians are trapped in eastern Syria.
Analysts have warned the USA decision to pullout troops from Syria will leave a vacuum that will "increase the worldwide and regional conflict" for power and influence in Syria.
A top United States commander has issued a warning saying there are "tens of thousands" of ISIS fighters spread across Syria and Iraq. "They have had the opportunity to prepare this for a while, and there is a presence of civilians in the area - family members of ISIS and then others that reside in the area, so I think this poses a significant, a significant concern for us as we do this", Votel said.
US officials have said in recent weeks that IS has lost 99.5 percent of its territory and is holding onto fewer than 5 square kilometers in Syria, or less than 2 square miles, where the bulk of the fighters are concentrated.
The displaced residents, mostly women and children, have been placed in the Kurdish al-Hol camp in al-Hasakah governorate, in northeast Syria.