The attack took place in the town of Baghouz on Thursday, leaving an unknown number of civilians including women and children dead, Press TV quoted the news agency as reporting.
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.
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.