Story highlights US-backed fighters want to wrest eastern parts of Deir Ezzor from ISIS

ISIS has faced a series of setbacks as territory shrinks in Syria and Iraq

(CNN) US-backed forces in Syria said Saturday they were launching an offensive against ISIS in the eastern city of Deir Ezzor.

The Syrian Democratic Forces, an alliance of mostly Kurdish and some Arab militias, said the push is designed to force the terror movement from eastern parts of the city -- the largest in eastern Syria and one of the largest in the country.

The SDF is already fighting ISIS in its de facto capital of Raqqa, Syria, and is now aiming to clear the eastern bank of the Euphrates River of the militant fighters, a statement said.

Deir Ezzor is arguably more important to ISIS than Raqqa.

It sits at the heart of Syria's oilfields, once an essential source of income to ISIS, and is Syria's main gateway to Iraq. It's also one of the most crucial in a string of towns along the Euphrates, where ISIS is expected to make its last stand.

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