WASHINGTON - The U.S. military coalition fighting the Islamic State group would welcome a concerted effort by the Syrian government or its Iranian-backed partner forces to defeat IS in its remaining strongholds in eastern Syria, a U.S. spokesman said Friday.

Army Colonel Ryan Dillon, spokesman for the coalition, told reporters at the Pentagon that the U.S. goal is to defeat IS wherever it exists. If others, including the Syrian government and its Iranian and Russian allies, want to fight the extremists as well, then "we absolutely have no problem with that," he said, speaking from Baghdad.

"If it looks like they are making a concerted effort to move into ISIS-held areas, and if they show that they can do that, that is not a bad sign," Dillon said, referring to forces of Syrian President Bashar al-Assad. "We are here to fight ISIS as a coalition, but if others want to fight ISIS and defeat them, then we absolutely have no problem with that."

Washington severed diplomatic relations with Syria during the Obama administration, which insisted that Assad "must go." More recently, Assad has strengthened his position, regaining key territory from weakened opposition forces.

Tight quarters

The battle space in Syria is getting more crowded and complex as IS-held territory shrinks, raising questions about how the various parties will interact with or avoid one another. Syrian government troops, for example, have reached the Iraqi border in an area where IS leaders have been gathering. The area is far from the main battle lines of Syria's civil war.

FILE - Syrian Democratic Forces (SDF) fighters. FILE - Syrian Democratic Forces fighters head to Raqqa, Syria, June 6, 2017. FILE - Syrian Democratic Forces fighters head to Raqqa, Syria, June 6, 2017.

The U.S. so far has shunned any cooperation with Assad and has partnered instead with local Arab and Kurdish forces in fighting IS. Those local forces, which the U.S. calls the Syrian Democratic Forces, or SDF, are currently fighting to recapture the extremists' self-declared capital of Raqqa.

Last weekend, for the first time, the U.S. shot down a Syrian fighter jet that had dropped bombs near the SDF. Two other times this month, the U.S. has shot down Iranian-made drones in southern Syria that were deemed to pose a threat to U.S. and partner forces.

Key remaining IS territory includes the cities of Deir el-Zour and Abu Kamal, along the Euphrates River valley.

Dillon said that as Syrian government forces move toward Abu Kamal, "if they want to fight ISIS in Abu Kamal and they have the capacity to do so, then that would be welcomed. We as a coalition are not in the land-grab business. We are in the killing-ISIS business. That is what we want to do, and if the Syrian regime wants to do that and they're going to put forth a concerted effort and show that they are doing just that in Abu Kamal or Deir el-Zour or elsewhere, that means that we don't have to do that in those places."