EDITORS NOTE: Graphic content / A picture taken on January 28, 2018, in the city of Afrin shows smoke billowing from surrounding villages during the Turkish military operation against the Kurdish enclave.
Turkey launched operation "Olive Branch" on January 20 against the Syrian Kurdish People's Protection Units (YPG) militia in Afrin, supporting Syrian opposition fighters with ground troops and air strikes. / AFP PHOTO / George OURFALIAN        (Photo credit should read GEORGE OURFALIAN/AFP/Getty Images)
PHOTO: GEORGE OURFALIAN/AFP/Getty Images
EDITORS NOTE: Graphic content / A picture taken on January 28, 2018, in the city of Afrin shows smoke billowing from surrounding villages during the Turkish military operation against the Kurdish enclave. Turkey launched operation "Olive Branch" on January 20 against the Syrian Kurdish People's Protection Units (YPG) militia in Afrin, supporting Syrian opposition fighters with ground troops and air strikes. / AFP PHOTO / George OURFALIAN (Photo credit should read GEORGE OURFALIAN/AFP/Getty Images)
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(CNN) —  

The US-led coalition fighting ISIS has observed pro-Syrian regime forces once again conducting a slow build-up east of the Euphrates River near where US troops are present advising local allies, according to multiple US and coalition officials.

The pro-regime forces are assembling near where similar forces allied with Syrian President Bashar al-Assad initiated an unprovoked attack on US-backed Syrian Democratic Forces, US Army Col. Ryan Dillon, a spokesman for the coalition told CNN.

Dillon added that the coalition was keeping a close eye on the activity and was communicating with their Russian counterparts via the de-confliction hotline to discuss the development.

He said the coalition hoped to “first deescalate the situation” but added “if it comes down to it we will defend ourselves.”

The coalition was also on the lookout to see if pro regime forces bring in any combat equipment, like artillery, that could threaten coalition troops.

A US military official told CNN that Russian private military contractors remain in the area and are using local pro-regime troops to scout coalition positions.

The slow buildup comes just days after pro-regime forces fired artillery rounds that landed about one kilometer from a US-allied force of Syrians operating near the coalition base in At Tanf, Syria.

“Coalition officials used the de-confliction line with Russian counterparts to deescalate the incident. No more rounds impacted in the area after the call,” Pentagon spokesman Maj. Adrian Rankine-Galloway told CNN.