"Despite incorrect information about changes to the military forces in Manbij city, (the US-led coalition) has seen no indication of these claims being true," US Central Command spokesman Lieutenant Colonel Earl Brown said on Friday.
Manbij is a strategic city close to the Turkish border where Kurdish forces have been deployed since 2016.
American and French special operations troops are also stationed there, assisting the Kurds, but the US military will be withdrawing under a surprise pull-out announced by President Donald Trump last week.
Brown called on all parties to respect the "integrity of Manbij and the safety of its citizens".
"Our mission has not changed. We will continue to support our coalition partners, while also conducting a deliberate and controlled withdrawal of forces, while taking all measures possible to ensure our troops' safety and that of our partners on the ground," he told AFP news agency.
'No sign of Syrian forces'
The Syrian government announced earlier on Friday that it had entered the town of Manbij and raised the national flag there.
It also pledged to guarantee "full security for all Syrian citizens and others present in the area", according to Syrian state-run news agency SANA.
However, Al Jazeera's Mohammed Adow, reporting from Gaziantep on the Turkish border, said residents of Manbij, which lies 30km south of the Turkish border, dispute the Syrian army's claim.
"Manbij residents who we spoke to have said that they have not seen any sign of Syrian forces in their city but what we know is that Syrian government troops have already been on the outskirts of the city, where they were part of an international coalition that is fighting remnants of ISIL," he said.
Nura al-Hamed, deputy head of the Manbij local authority, told AFP that the regime deployment was the result of Russian-sponsored negotiations.
"The regime forces will not enter the city of Manbij itself but will deploy on the demarcation line" with Turkish-backed Syrian groups, she said.
Shift in alliance
The Syrian army's deployment creates a government buffer arching across northern Syria that fully separates the Turkish army and its proxies from the Kurds.
Turkey reacted to the deployment by warning "all sides to stay away from provocative actions" while a large convoy of its Syrian auxiliaries was seen moving closer to the western edge of Manbij.
The US withdrawal from Syria has sent Kurdish forces scrambling to find allies to fend off a possible attack from Turkey, which views the fighters as "terrorists".
The Kurds have welcomed a Syrian government advance in Manbij province, a pragmatic shift in alliances that will dash their aspirations for autonomy but could help them cut their losses.
The People's Protection Units (YPG) have been the backbone of an alliance that has spearheaded the US-backed fight against the Islamic State of Iraq and the Levant group (ISIL, also known as ISIS) in Syria.
"We invite the Syrian government forces... to assert control over the areas our forces have withdrawn from, particularly in Manbij, and to protect these areas against a Turkish invasion," the YPG said in a statement.
Al Jazeera's correspondent said the YPG's appeal was a "tactic by its fighters to avoid confrontation with the Turkish forces who they, of course, know they are no match for".
Following the conflicting reports regarding Manbij on Friday, Turkish President Recep Tayipp Erdogan said Turkey will have no reason to be in Manbij once "terrorist organisations" leave.