The number of Guantanamo inmates on hunger strike has grown from 15 to 26 in a week.
The protest, entering into its seventh week, is over indefinite detention at the prison and the diminishing prospect that it will close.
Feeding tubes have been administered to eight inmates, according to military authorities at the US naval base in Cuba.
The strike was launched on February 6, when during a routine inmate search, guards inspected Korans, which was perceived to be religious desecration.