The attack took place in the town of Baghouz on Thursday, leaving an unknown number of civilians including women and children dead, it added.
The so-called Syrian Democratic Forces (SDF), which is supported by Washington, claims that is fighting the last remaining pocket of land controlled by Daesh terrorists in Baghouz.
Thursday’s bombing marks the third reported US attack with white phosphorus bombs after the SDF announced it had resumed operations following a break in the fighting to facilitate civilian evacuation last week.
Women and children have been reportedly killed in a chemical attack by the US-led coalition against a Syrian city.
Washington has long been providing the SDF — a predominantly Kurdish alliance of militants — with arms and militants, calling the group a key partner in the purported fight against Daesh.
The US and its allies have been conducting airstrikes against what they call Daesh targets inside Syria since September 2014 without any authorization from the Damascus government or a UN mandate.
Damascus has repeatedly denounced the US military presence in the country and its aerial assaults which have repeatedly targeted civilians and its critical infrastructure.
The Syrian Foreign Ministry has called on the UN Security Council to stop US-led coalition airstrikes on civilian infrastructure in the crisis-hit country.
The US has previously used internationally banned munitions in Syria, particularly in the Dayr al-Zawr province.
In June 2017, the Human Rights Watch (HRW) warned that the US-led coalition was deploying white phosphorous bombs in both Iraq and Syria.