Although infant mortality rates in the United States are still edging down, the decline has slowed sharply in recent years, with local rates in many large cities such as Washington as much as twice the national average, the Food Research and Action Center (FRAC) reported yesterday.

In a study of infant mortality, the nonprofit advocacy group also found that "the disparity in low birth weight and infant mortality rates between nonwhite and white Americans is great, and from 1979 to 1984 this gap increased in most of the large U.S. cities and many of the states." Low birth weight is a major cause of infant death.

Looking at the 54 largest cities in 1984, when the national figure was 10.8 per 1,000 live births, FRAC found that the District of Columbia had the highest infant mortality rate, 21.2 per 1,000 live births (7.8 for whites but 24 for nonwhites).

The next highest rate was in Detroit (21), followed by Atlanta (19.3), Newark, N.J. (18.6), Cleveland (16.9), Norfolk (16.7) and Baltimore (16.6). New York, at 13.6, ranked 18th and San Francisco, at 8.8, was among the lowest. The lowest rates were in Oklahoma City (5.6) and Tulsa (6.3).

The District figure declined to 20.7 in 1985 but rose slightly to 21 last year, according to the D.C. Office of Maternal and Child Health. Many local initiatives have been proposed by city officials in recent years to bring the rate down.

Robert J. Fersh, FRAC executive director and former staff director of the House subcommittee on nutrition, said, "There are many contributing factors to the high urban rates but the overriding issue is poverty, the lack of money to purchase food, to receive proper medical care. Seven of the 10 cities with the highest infant mortality rates were among the 10 poorest cities."

The report called for improvements in federal nutrition and health programs for low-income pregnant women and children.

In 1960, according to government statistics, the national infant mortality rate was 26 per 1,000 live births. It reached 12.6 in 1980. But it then declined very slowly, reaching 10.8 in 1984 and 10.4 for 1986. Rates for blacks have been almost twice as high as the national rate.