Come again? Do you mean pure fusion thermonuclear devices? Fission weapons were the primeval kind of nuclear weapons ever built.
Why the nation should be "developing fission weapons" (when it already has them - since as early as 1974 with the polonium-beryllium "Smiling Buddha") is a gratuitous (perhaps, accidental) question.
With the 45 kiloton Pokhran-II detonation in 1998, India received its first potential fusion/ boosted weapon device. The Shakti-I infact, was a two-stage thermonuclear device with fusion boosted primary, intended for a missile warhead, with a 200 kt deployed yield. Therefore, a fission-fusion nuclear device is already in India's posession- for well over a decade.
Pure fusion weapons or hypothetical hydrogen bombs have been the unrequited dream of scientists for decades. It would require the removal of the fission primary (or elementary stage) required to ignite the fusion of deuterium and tritium. Inducing a fusion reaction requires a high power density, the kind that can only be initiated by powerful laser devices, magnetic confinement devices or fission reactions, as in current multiple-stage thermonuclear weapons. To date however, they remain only a theoretical possibility.
Some cursory research reveals that in 1998, the U.S. Department of Energy released a restricted data declassification decision stating that "the U.S. does not have and is not developing a pure fusion weapon and no credible design for a pure fusion weapon resulted from the DOE investment".