The Marine Corps, which has encountered difficulty in proving charges against members of its U.S. Embassy detachment in Moscow, said yesterday another former member of the unit has been charged with copying classified U.S. documents in Switzerland and forwarding them to a private residence in a Chicago suburb.

Sgt. Kenneth J. Kelliher, 32, of Hinsdale, Ill., became the sixth person charged in a widespread military investigation into allegations that the Moscow guards traded secrets for sex with Soviet women.

Charges against three of the Marines have been dismissed or reduced for lack of corroborating evidence or because of jurisdictional problems.

Kelliher, accused of copying the documents at the U.S. Embassy in Bern, was also charged with failing to report three contacts with Soviet women while stationed in Moscow from September 1984 until March 1986.

The corps said Kelliher is charged with engaging in black-market activities for a Soviet woman, identified as Anna Novikoff, by purchasing items for her through the U.S. mail.

Marine Corps officials have maintained that black-market dealing by guards at the Moscow embassy has been relatively small and involved no Soviets and that the few resulting cases have been handled by transfers and administrative punishments.

Kelliher was ordered to face an Article 32 hearing, the military equivalent of a pretrial hearing, Monday at Quantico Marine Base, where he is stationed.

Among charges growing out of his Swiss duty is an allegation that he allowed a Swiss woman, identified as Regula Sommerhalder, into "an unauthorized area of the Bern embassy after hours."

Similar charges that Sgt. Clayton J. Lonetree of St. Paul, Minn., and Cpl. Arnold Bracy of Queens, N.Y., allowed Soviet intelligence agents into the Moscow embassy were dismissed after military authorities said prosecutors could not prove the allegations.

The charges disclosed yesterday did not mention Lonetree nor Bracy, who were in Moscow when Kelliher was.

Marine officials declined to elaborate on the accusations.