The activist, Sam Sokha, was arrested after she was returned to Cambodia on Thursday.
Police said she was to serve a two-year prison sentence after being convicted in absentia on January 25 on charges of insulting a public official and "incitement to discriminate."
State Department spokeswoman for East Asia, Katina Adams, said Monday the US was "deeply concerned" that Sam Sokha was repatriated in spite of her UN-recognised refugee status.
Adams urged Thailand to refrain from "involuntary refugee returns."
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