Moslems are the fastest growing population group and may account for nearly one quarter - 1.9 billion people - of the world's population by 2025, a new study said Thursday.

The study, "The Demography of Islamic Nations," was written by John Weeks and Saad Gadalla for a non-profit research group, the Population Reference Bureau, and put the current world Moslem population at 984 million people."Among the five most populous nations, the United States has by far the fewest Moslems," the report said. It said estimates for the number of Moslems in the country varied widely, adding, "It is probably reasonable to assume that, as of 1988, the Moslem population was somewhere between 1.5 million and 6 million."

The report said there are 40 nations in the world in which a majority of the population is Moslem and an additional seven countries in which a significant minority - 25 percent to 49 percent - of the population is Moslem.

Regionally, the report said that the 22 Middle East nations of North Africa and Western Asia have a Moslem population of 309 million, 31 percent of the population of Islamic nations.

Sub-Saharan Africa, with 17 Islamic nations, has 225 million Moslems, nearly 26 percent of the population of Islamic nations while Southern Asia holds 232 million people in four nations.

Only one European nation - Albania - has a majority Moslem population, the report said. Indonesia is the most populous nation with a majority Moslem population, with 88 percent of its 177 million people estimated to be Moslem.

"Although Islam is a proselytizing religion, its proportionate increase in the modern world is much more a result of natural increase - the excess of births over deaths - than it is the conversion of non-Moslems to the Islamic faith," the report said.