Pentagon spokesman Navy Captain Jeff Davis says the US wants to see focus on containing Daesh as multiple powers vie for operation in Manbij and beyond.
Russian and Syrian regime forces are moving "humanitarian" convoys carrying military equipment into Manbij in northern Syria, the US Department of Defense said on Friday.
"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 armoured equipment with them," Pentagon spokesman Navy Captain Jeff Davis said.
He said the military equipment included "up-armoured vehicles." Another defence official said the material appeared to be for "force protection."
Manbij has become the centre of geopolitical intrigue extending beyond the fight against Daesh, as multiple international powers with competing goals hone in on the area.
The United States is backing the Syrian Democratic Forces (SDF), which pushed Daesh from Manbij last year. The SDF is dominated by the YPG, the armed wing of the PYD, the Syrian affiliate of the PKK which is recognised as a terrorist organisation by Turkey, the US, and the EU.
Turkey is also carrying out an operation in northern Syria with the Free Syrian Army (FSA) against Daesh. In addition, Turkey has been fighting outfits affiliated with the PKK.
On Thursday, Turkey threatened YPG to leave the city of Manbij, and has repeatedly told the US that it will not work with the YPG.
The same day, YPG said it was going to hand over villages near Manbij to the Syrian regime under a deal agreed with Russia. These villages are controlled by the Manbij Military Council which is part of YPG.
The Turkish military campaign in Syria known as Operation Euphrates Shield launched in August last year. Turkish military forces took over the Syrian town of Jarablus, clearing Daesh fighters from a roughly 100 kilometre stretch of the border before moving on to al-Bab, a strategic town now all but secured.
Russian and Syrian regime troops entering Manbij might be beneficial for the United States, as it could prevent Turkish and YPG 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 ground focus [on Daesh]," Davis said. He added that Russia had informed the US about the convoys ahead of time through a special hotline.
Meanwhile, tens of thousands of Syrian civilians have fled as Russian-backed regime forces have been fighting Daesh in the north over the past week, a war monitoring group said on Saturday.
More than 30,000 civilians have left Aleppo province. Most of them were women and children, according to the Britain-based Syrian Observatory for Human Rights.