U.S.-backed fighters are closing in on the “last pocket of ISIS resistance” in Syria, and remaining ISIS fighters are becoming increasingly desperate, the U.S. military said Tuesday.

The final offensive against ISIS in Syria is dubbed “Operation Roundup” and was started three weeks ago by the Syrian Democratic Forces, or SDF, a coalition of Arab and Kurdish fighters trained, equipped, and advised by the U.S.

“As ISIS retreats, they're employing improvised explosive devices along routes and booby-trapping houses to inflict casualties as the SDF fighters clear through the terrain,” said Col. Sean Ryan, a U.S. military spokesman in Baghdad.

He said ISIS has been firing mortars indiscriminately in an attempt to distract the pursuing forces.

“These tactics clearly demonstrate the growing desperation of ISIS as the SDF slowly closes in on the last group of remaining fighters in Syria," he said.

The U.S. says the ISIS fighters have nowhere to run, and are methodically being captured or killed the anti-ISIS fighters.

More than 2,300 men, women, and children have been taken into custody by the SDF forces, including about 700 adult male combatants.

The fighting is centered in Syria’s Middle Euphrates River Valley, or MERV, near the Iraqi border in the far east of the country.

In the past week, two more cities have been liberated by the SDF as they push northward.

“Let me be clear: ISIS is not yet defeated,” said Ryan, adding that ISIS fighters are “in the final throes of their evil ambitions.”