He said the military equipment included "up-armoured vehicles".
Turkish forces and their rebel allies drove Islamic State (IS) from al-Bab last week.
The situation is similarly complicated on al-Bab's eastern front, where Turkish-backed rebels are battling USA -backed Syrian Democratic Forces (SDF), a Kurdish-led coalition force.
A USA -allied militia in northern Syria said on Thursday it had agreed to hand over villages on the front line with Turkey-backed rebels to Syrian government control under an agreement with Russian Federation.
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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 USA allies - from fighting there. 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 USA, and the EU.
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. But Turkey continues to assert that the group remains in the city.
Capt. Davis said SDF didnít inform them about the agreement, and didnít have much information about this development, but he encouraged all parties to focus on fighting Daesh. Turkish government sees Manbij as a natural target to seize due to protect its borders against terror group PKKís Syrian armed wing YPG. "We stated earlier that Turkey will hit the YPG unless they withdraw from Manbij".
Davis said the United States has noticed and observed these humanitarian convoys moving into the area. "The number, the size of them, and how many Kurdish units are participating in that, I can't really say right now".