The unlikely prospect of US and Russian armored vehicles rumbling down the same streets is another bizarre development in Syria's tangled conflict that has raged for six years.
"They can observe each other's movements," US military spokesman Colonel John Dorrian told reporters.
"They can see each other. They are not talking to each other, and they are not hanging out together."
A few dozen Army Ranger special operations forces earlier this month entered Manbij on a "reassurance and deterrence" operation in which the normally low-profile troops drove American flag-flying convoys through the city.
Their presence creates a buffer of sorts between Syrian Kurdish forces and Turkish troops eyeing the city, and the Pentagon has said that it wants all parties to focus on fighting IS jihadists in the region -- and not each other.
The United States is backing a Kurdish-Arab alliance called the Syrian Democratic Forces (SDF), whose 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, whom it views as terrorists.
Signup to our newsletter and follow us on Facebook and Twitter!
Turkey threatened to strike the Syrian Kurdish militia forces unless they withdrew from Manbij.
Russia and Syrian regime forces have since moved humanitarian convoys carrying military equipment into the city.
Dorrian said the US and Russian troops do not communicate on the ground, but keep each other abreast of their movements through a special hotline.
This channel was initially set up in 2015 to prevent mishaps in the air as Russia started bombing Syria on behalf of President Bashar al-Assad.
The developments come as Syria marks the sixth anniversary of its brutal conflict, which began with anti-government protests in March 2011 and has killed more than 320,000 people.
The civil war and the anti-IS fight are interconnected but for the most part have played out in separate parts of Syria.
"The Syrian civil war –- one of the world's worst humanitarian crises since World War II -– continues to be the most dangerous and destabilizing conflict in the Middle East today," Democratic Senator Tim Kaine said in a statement.
"Six years in, Assad has created a safe haven for (IS) and Al-Qaeda, while he commits unspeakable acts of violence against his own people."