Syria's armed Forces has entered into manbij after they were invited by Kurdish armed group to take control of the northern area and protect it from a threatened Turkish offensive.
State-run SANA news agency said the Syrian army raised the national flag in Manbij on Friday.
It also pledged to guarantee "full security for all Syrian citizens and others present in the area", according to SANA.
Manbij residents which lies 30km south of the Turkish border, disagrees with 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.
A senior Kurdish official reportedly said that government troops have arrived not entered the city.
Ilham Ahmed said the US troops who patrol the city and tense front line with Turkey-backed fighters have not withdrawn from Manbij, adding that an agreement is being worked out with the Russians and the Syrian government to allow the government to take over in case of a full US withdrawal.
"The aim is to ward off a Turkish offensive," Ahmed said.
"If the Turks' excuse is the [Kurdish fighters], they will leave their posts to the government," she said.
The military declaration came moments after the People's Protection Unit, or YPG, made an appeal to President Bashar al-Assad's government to prevent a "Turkish invasion".
The YPG, which Turkey regards as a "terrorist" group, said its fighters had previously withdrawn from Manbij to fight the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).
With the YPG at its forefront, Syrian Democratic Forces (SDF) seized Manbij in 2016 from ISIL, a milestone in the US-backed battle against the armed group.
The Manbij Military Council, fighters allied to the SDF, hold the city in northern Syria, which lies on a front with Turkey-backed rebels.
Turkey had threatened a military operation against Manbij to remove the Kurdish-led forces there.
Turkey and its allied fighters have been amassing troops around the city in recent days.
Our 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.
The Turkish threats triggered the US announcement that it would withdraw troops from Syria. A timetable for the withdrawal has not yet been made public.
But the surprise US decision rattled allies and the US Syrian Kurdish partners, who scrambled to find new allies to protect their Kurdish-administered areas in northern Syria.
Assad's government has said it welcomes the Kurdish group returning under its authority.
But government officials have stated they will not consider an autonomous area, a main demand for the Kurds. There was no immediate response from the United States.
Russia welcomed the Syrian army's entry into Manbij. "Of course, this will help in stabilising the situation.
The enlargement of the zone under the control of government forces ... is, without doubt, a positive trend," said Kremlin spokesman Dmitry Peskov.
Sami Nader, director of the Levant Institute for Strategic Studies, said Syrian troop entry into Manbij, if confirmed, was a "significant development".
"It is a solution all regional powers can accept because Turkey doesn't want US troops in Manbij, while the other players don't want Turkish troops there."
Nader also said that The YPG invitation was a "last resort because they are squeezed and they fear an Afrin scenario", he said, referring to a Turkish assault earlier this year which expelled the YPG from the border town of Afrin.