WASHINGTON (Reuters) - U.S.-backed militias fighting Islamic State are not planning to enter the city of Deir al-Zor in eastern Syria, a U.S. military spokesman said on Thursday, reducing the chances of the American allies coming face to face with Syrian government forces. The Syrian army and its allies recently broke an Islamic State siege of an enclave in Deir al-Zor that had lasted three years. The Syrian Democratic Forces (SDF), a U.S.-backed alliance of mostly Arab and Kurdish fighters, meanwhile have also launched a separate assault in Deir al-Zor province, which has become Islamic State s last major foothold in Syria. Earlier this week, the SDF said it had reached an industrial zone miles to the east of Deir al-Zor city. The advances bring the SDF to within 15 km (10 miles) of the Syrian army. The advances against Islamic State, another blow to its control over territory it held for years as part of a self-declared caliphate, has raised concerns that it would bring the SDF into closer proximity to the Syrian government side, which is backed by Russia and Iran.  U.S. Army Colonel Ryan Dillon, a spokesman for the U.S.-led coalition fighting Islamic State, told reporters that the battlefield was already congested and the plan was for the SDF to move down the middle Euphrates River Valley.  I ll just tell you that the plan is not to go into Deir   al-Zor city but there (are) plenty of ISIS fighters and resources and leaders that continue to have holdouts throughout the middle Euphrates River valley,  Dillon told reporters in Washington through a video briefing. 