Eastern Syria (CNN) Heavy fighting erupted Sunday as US-backed forces attacked the last ISIS stronghold in Syria amid a final push to clear the jihadist group from the war-torn country.

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.

Men suspected of being ISIS fighters wait to be searched by the Kurdish-led Syrian Democratic Forces after leaving Baghouz last month.

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.

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