I am trying to cite this document but I found out that, apart from informal in-text citations such as 'according to x' (which I can cite as a secondary source because I was never successful in locating the original primary source), other information are not cited/referenced and there's no reference list. Can I rely on such information to cite the report? Why don't they provide references I can track for their claims? Do they consider such information as part of common knowledge?
There is no single such thing as "a US government report": the US government is a vast and complex organization with lots of different elements writing lots of different types of reports for lots of different reasons. These reports range from absolutely standard peer-reviewed scientific papers to high-quality primary data to informational digests to purest propaganda. As such, you really need to specifically attempt to understand the context of a particular report in order to know how to interpret it.
In the particular instance that you cite, for example, it appears that this is part of a yearly assessment prepared for Congress by the US State department as a digest of primary source material gathered by various US embassies. Thus, I would recommend considering it as primary source material which attempts accuracy but not the results of a scientific study. You will have to assess for yourself what credence to lend to particular elements thereof with respect to the use you wish to put it to.