CAIRO — Security forces have detained an Egyptian employee of the United States Embassy who worked as a liaison to the Muslim Brotherhood, Egyptian news reports said Wednesday, stirring fears of pressure on Western diplomats who communicate with the Islamist opposition.

Embassy officials said the employee, Ahmed Alaiba, was detained on Jan. 25, the third anniversary of the Arab Spring uprising here, and he has been held without charges since then. Egyptian news media have reported anonymous statements from Egyptian security officials that he had participated in “rioting” during demonstrations that day against the military ouster last summer of President Mohamed Morsi of the Muslim Brotherhood.

An Egyptian government official briefed on the case said Mr. Alaiba was under investigation for both participating in an illegal demonstration and “communicating with an outlawed group.”

The pro-government Egyptian news media often accuse the United States of conspiring with the Brotherhood, which the new military-backed government has outlawed as a terrorist group. Reports of Mr. Alaiba’s arrest have included accusations that he acted as a link with the Brotherhood leadership.