An Immigration and Naturalization Service supervisor and four INS detention officers were indicted yesterday by a federal grand jury in Fort Smith, Ark., on charges of assaulting Cuban refugees during disturbances at nearby Fort Chaffee last fall.

The 18-count indictment alleges that INS supervisor Curtis A. Clark told his subordinates to beat the refugees during processing and later ordered a cover-up by getting an assault allegation erased from a medical file. It also describes how two of the guards allegedly tried to intimidate the Cubans by introducing themselves as "Official de la Muerte" (Officer of Death) and "La Planadora" (The Steamroller).

Twelve Cubans were beaten by the officers last September and October, according to the charges. In one instance, a refugee on a hunger strike was "pulled up by his hair and dropped to the ground, kicked, struck with riot batons and sprayed with chemical Mace."

Federal charges against federal law enforcement officers are unusual, though several Border Patrol officers were convicted last year of beating Mexican aliens who crossed the border near San Diego.

The Cubans allegedly attacked were part of the flood of refugees that entered the United States last spring. The INS officers were charged with an assault on a federal reservation, rather than the usual civil rights laws, and the investigation is continuing, according to James P. Turner, acting head of the Justice Department's Civil Rights Division.