BEIRUT — U.S.-backed and Kurdish-led Syrian forces said Saturday they have launched a final push to defeat the Islamic State in the last tiny pocket the extremists hold in eastern Syria.

Syrian Democratic Forces spokesman Mustafa Bali tweeted that the offensive began Saturday after more than 20,000 civilians were evacuated from the IS-held area in the eastern province of Deir el-Zour. An SDF statement said the offensive was focused on the village of Baghouz.

The SDF, backed by U.S. air power, has driven IS from large swaths of territory it once controlled in northern and eastern Syria, confining the extremists to a small pocket of land near the border with Iraq.

Scores of IS fighters are now besieged in two villages, or less than one percent of the self-styled caliphate that once sprawled across large parts of Syria and Iraq.

In recent weeks, thousands of civilians, including families of IS fighters, left the area controlled by the extremists.

“The decisive battle began tonight to finish what remains of Daesh terrorists,” Bali said, using an Arabic acronym to refer to IS.

“The battle is very fierce,” he later said. “Those remaining inside are the most experienced who are defending their last stronghold. According to this you can imagine the ferocity and size of the fighting.”

Bali did not say how long they expect the battle to last.

The Britain-based Syrian Observatory for Human Rights, a war monitor, said SDF fighters are advancing “cautiously” because of mines planted by IS gunmen.

It also said U.S.-led coalition warplanes are giving cover to advancing SDF fighters.

President Trump predicted Wednesday that IS will have lost all of its territory by next week.

“It should be formally announced sometime, probably next week, that we will have 100 percent of the caliphate,” Trump told representatives of the 79-member, U.S.-led coalition fighting IS.

U.S. officials have said in recent weeks that IS has lost 99.5 percent of its territory and is holding onto less than 2 square miles in Syria, where the bulk of the fighters are concentrated.

But activists and residents say IS still has sleeper cells in Syria and Iraq, and is laying the groundwork for an insurgency.

The U.S. military has also warned that the group could stage a comeback if the military and counterterrorism pressure on it is eased.