Those accused include both Cambodians and Cambodian-Americans, mostly employed as guards and a few as clerical staff.
None of the accused are diplomats, according to sources who spoke to Reuters on condition of anonymity.
Images seen by the wife of an embassy worker were reported and forwarded to the FBI, the sources told Reuters.
The message group reportedly contained both photographs and videos, and some of the material featured subjects under 18 years of age.
Child rape and sex trafficking remain serious problems in Cambodia, a Southeast Asian nation emerging from decades of poverty and internal conflict.
The State Department has regularly highlighted sex trafficking and child sex tourism in its annual trafficking in persons report.
The dismissals come at a time of tension between the governments of the U.S. and Cambodia.
Washington has criticized Prime Minister Hun Sen’s recent crackdown on democratic opposition.
Cambodia has in turn accused the U.S. of interfering in the country’s politics and attempting to undermine its leadership.