Washington (CNN) Pro-Syrian regime forces have begun massing east of the Euphrates River near where US troops are present while advising local allies, according to three US military officials. They are assembling near where similar pro-regime forces allied with Syrian President Bashar al-Assad initiated an unprovoked attack on US-backed Syrian Democratic Forces, the officials said.

One US military official said that the coalition had used the pre-established de-confliction hotline to raise the increased pro-regime presence with the Russian military.

The US military is monitoring the situation in Deir Ezzor to see if the pro-regime troops attempt to once again attack areas controlled by the SDF.

"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," Pentagon spokesman Maj. Adrian Rankine-Galloway told CNN.

"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," he added.

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