"Accepting even small amounts of money could be construed as "material support", according to the senior director of the Counter Extremism Project, Hans-Jakob Schindler.
Under US law, X is legally obliged to refrain from providing services to sanctioned groups in exchange for payment, and must promptly report illicit transactions to appropriate authorities.
"While some steps seem to have been taken to address this issue, X's belated response to this oversight, and only in the face of public scrutiny, is insufficient", Mr Schindler said.
The company's actions remain "deeply concerning and demonstrate an apparent disregard for its legal obligations", he added."