A CNN team in Syria witnessed airstrikes, explosions and mortar fire in Baghouz after the operation by the Syrian Defense Forces began about 6 pm local time.
SDF spokesman Mustafa Bali said on Twitter Sunday that its forces were engaged in “direct violent clashes” and that its planes were targeting weapons depots.
Earlier Sunday, Bali tweeted that “the timeline … for ISIS to surrender themselves is over” and that SDF forces were ready to “finish what is left in ISIS hands.”
At its height, ISIS controlled huge swaths of territory in Syria and Iraq. The US-led coalition has been working for years to oust the group from cities and towns.
It was not clear Sunday how many fighters and civilians remained in the half-mile-square pocket under ISIS control in Baghouz.
Before the recent offensive started last month, SDF officials estimated that 1,500 civilians and 500 ISIS fighters remained, but as the assault got under way it became clear that the actual number was much higher.