Is a Masters Degree mandatory to gain entrance to a PhD in US I have been working as a Research Assistant in a university for 2 years under the supervision of a supervisor who has a doctoral degree. 

I have fair knowledge about research and a first class for my Bachelors Degree. I wish to apply for a PhD in United States. 

Is it possible to secure a PhD opportunity without a Masters Degree?
No, in a rough manner...

There are a multitude of Ph.D. programs, presented by US universities, within which the M.Sc. degree would not be as a mandatory factor for entrance the target program.

As a matter of fact, direct transition from B.Sc. (first-class or 4-year one) to Ph.D. would be a research plan for the future action within the academia, where one often seeks a M.Sc. degree for the participation within the industry.

All in all, you better check the admission requirements of the target programs, by case, to realize the necessity...