Fight against ISIS in eastern Syria slowing down as some Kurds leave battlefield Pro-Regime forces are also massing again close to scene of attack in February.

Offensive ground operations against ISIS in eastern Syria have slowed down as hundreds of Kurdish fighters have left the battlefield to potentially assist Kurdish forces in Afrin, in western Syria, who are fighting the Turkish military.

Meanwhile, the U.S. is monitoring the build-up of a pro-Assad regime force massing in the same area, where in February, another similar force attacked Kurdish and U.S. forces. U.S. airstrikes used to repel that attack killed up to 100 pro-regime fighters, including dozens of Russian private military contractors. CNN was first to report the massing of the pro-Assad regime force.

Since retaking Raqqa from ISIS last fall, the U.S-backed Syrian Democratic Forces (SDF) have pushed the shrinking number of ISIS fighters further eastward along the Euphrates River towards the border with Iraq. The SDF is a Kurdish-led force of 60,000 anti-ISIS fighters evenly split between Syrian Kurdish and Arab fighters.

As the military fight against ISIS has wound down the shrinking battlefield in eastern Syria has become more complex as the risks increase that Kurdish, American, Russian and Assad military forces could come into potential conflict.

In late January tensions increased as Turkey launched a military operation into the Kurdish-held area of Afrin in western Syria. Turkey said its military operation was targeting the YPG that it has branded a Kurdish terrorist force. The YPG make up a large number of the Syrian Kurdish forces fighting for the U.S.-backed Syrian Democratic Forces that are fighting ISIS.

In February, small numbers of Kurdish SDF fighters who had been fighting ISIS in eastern Syria headed westward to help Kurdish forces in Afrin.

At the time U.S. officials said the movements were not having an overall effect on the SDF's military campaign against ISIS.

But that changed last week as the Pentagon acknowledged the SDF was carrying out an “operational pause” in eastern Syria.

According to U.S. officials, a contributing factor for the pause was that the number of SDF fighters who had left the fight in eastern Syria had swelled to 1,500. That included Arabic SDF fighters who have been doing the bulk of the fighting against ISIS in eastern Syria. But they left the fight against ISIS as they lost the battlefield support provided by the better trained Kurdish forces who had moved west.

Most of the Kurdish SDF fighters who left the eastern battlefields have headed towards the western city of Manbij that is close to Afrin. U.S. officials do not have a good grasp on how many Kurdish fighters may have actually moved from Manbij towards Afrin.

On Tuesday, the Syrian Democratic Forces formally said in a statement that it was moving its forces from Dayr Az Zawr Province, in eastern Syria, towards Afrin to counter the Turkish incursion.

A U.S. official said another contributing factor to the significant decrease in the SDF's offensive operations against ISIS was a shift towards a defensive posture to maintain the SDF's major territorial gains in eastern Syria.

Also in eastern Syria a U.S. official says the U.S. is monitoring the buildup of a pro-Assad regime force in the same area as the unprovoked attack on February 7 against a SDF base.

U.S. military advisers working at the headquarters called in airstrikes to repel a force of 300 to 500 pro-regime forces. About a hundred pro-regime fighters were killed in the fighting, including dozens of Russian military contractors.

According to the U.S. official the pro-regime force has been gathering on the eastern side of the Euphrates River, the dividing line between U.S.-backed and Russian-backed pro-regime forces.

The U.S. military has been using the phone hotline to the Russian military to raise concerns about the group. The contacts mirror those made prior to the February 7 attack with the Russian military who said there were no Russian military personnel in the area.

A Pentagon spokesman stressed that coalition forces are prepared to defend themselves if needed.

"While we do not seek confrontation with pro-regime forces, as we have demonstrated repeatedly, Coalition forces will not hesitate to protect themselves when they are threatened," said Major Adrian Rankine-Galloway.

"Coalition forces maintain good situational awareness of military developments throughout Syria and take appropriate force protection measures to ensure our forces are safe and can carry out their combat missions to rid Syria of ISIS."