Russia and Syrian regime forces are moving "humanitarian" convoys carrying military equipment into the former Islamic State group's bastion of Manbij in northern Syria, the US Defense Department said Friday.
The move would likely make it more difficult for Turkish troops to strike Syrian Kurdish forces if they did not withdraw from Manbij.
"We have noticed and observed and are aware of the fact that these humanitarian convoys sponsored by the regime (of President Bashar al-Assad) and Russia have been moving into Manbij and that they have some armored equipment with them," Pentagon spokesman Navy Captain Jeff Davis said.
He said the military equipment included "up-armored vehicles" and another defense official said the materiel appeared to be for "force protection."
Manbij has become the center of geopolitical intrigue extending beyond the fight against the Islamic State, as multiple international powers with competing goals home in on the area.
The United States is backing a Kurdish-Arab alliance called the Syrian Democratic Forces (SDF) to fight IS, and these fighters pushed the jihadists from Manbij last year.
Around the same time, Turkey crossed into northern Syria and joined the anti-IS fight, while also working to keep in check the Kurdish fighters, which it views as terrorists.
Turkey threatened Thursday to strike the Syrian Kurdish militia forces if they did not withdraw from Manbij.
Russian and Syrian regime troops entering Manbij might be beneficial for the United States, as it could stop Turkey and Kurdish forces -- both of whom are US allies -- from fighting there.
"All I would say is we just continue to want to see all parties on the ground continue to focus (on IS)," Davis said.
He added that Russia had informed the United States about the convoys ahead of time through a special hotline.