ALBANY — A couple accused of kidnapping and sexually abusing two young Amish sisters in August in upstate New York are facing additional federal charges involving more child victims, prosecutors said on Wednesday.

The case against the couple, Stephen Howells and Nicole Vaisey, now involves six victims, said Richard S. Hartunian, the United States attorney for the Northern District of New York.

Mr. Howells and Ms. Vaisey, who live in Hermon, N.Y., had previously been charged with crimes against three children, including the Amish girls.

The new 21-count indictment had not been filed as of early Wednesday evening, so details were not available. But Mr. Hartunian said the couple was charged with sexually exploiting three children from September 2013 to August 2014, and that Mr. Howells is additionally charged with six counts involving four children at other times.