The so-called Syrian Observatory for Human Rights (SOHR) said the base is located in the al-Jazrat area in the western countryside of Deir Ez-Zor after "massive" supplies and reinforcement reached that area, Xinhua reported.
The group said 300 military shipments of the US had reached al-Jazrat over the past few days.
It added that the US forces are also expanding their base in the al-Omar oil field in the eastern countryside of the province.
The US has several bases in Syria, mainly in northern and eastern regions where the Kurdish-led Syrian Democratic Forces (SDF) are located.
The Syrian government has for long demanded the withdrawal of the US forces from Syria, branding them as forces of occupation.
However, the US is consolidating its positions in Syria, mainly in areas where oil fields are located.