Damascus, Mar 11 (IANS): Heavy fighting took place as US-backed forces attacked the last Islamic State (IS) stronghold in Syria amid a final push to clear the terror 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 Defence Forces (SDF) began at about 6 p.m. on Sunday.

SDF spokesman Mustafa Bali said on Twitter that its forces were engaged in "direct violent clashes" and that its planes were targeting weapons depots.

Before the fighting began, Bali tweeted earlier Sunday that "the timeline... for IS to surrender themselves is over" and that SDF forces were ready to "finish what is left in IS hands".

At its height, the IS 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 on Sunday how many fighters and civilians remained in the half-mile-square pocket under IS control in Baghouz.

Before the recent offensive started last month, SDF officials estimated that 1,500 civilians and 500 IS fighters remained, but as the assault got under way it became clear that the actual number was much higher.

On March 5, about 500 IS fighters surrendered to SDF forces.

The militants were among a wave of people fleeing fighting in the village, near the Iraqi border, which has been pummeled by US-led coalition airstrikes in recent weeks.