The District of Columbia's rate of infant deaths rose a startling 16.5 percent last year after three years of steady decline, according to figures released yesterday, alarming Mayor Marion Barry and other officials.

Barry, who has boasted for the last few years that city outreach programs had finally begun to pull down the city's notoriously high infant mortality rate, said he was at a loss to explain why the situation had worsened, but suspects it is related to high levels of alcohol and drug abuse in the District.

"The increase in the District's rate has caused us alarm, although we realize that many of the contributing causes to infant mortality in general are socioeconomic in nature," Barry said at his monthly press conference.

The infant mortality rate here increased from 18.2 deaths per 1,000 live births in 1983 to 21.2 deaths per 1,000 in 1984 -- one of the worst rates of any major city in the country. In 1984, a total of 9,549 babies were born here and 202 infants under 1 year old died, compared with 9,524 births and 173 deaths in 1983.

"That [increase] doesn't sound like a fluke," said Kay Johnson of the Children's Defense Fund, a nonprofit child advocacy group that has studied infant mortality. "Given the trend of the District over time, it's more likely the improvement in recent years was a fluke."

Health experts say there is a strong correlation between poverty and infant mortality and point to a wide gap between black infant mortality rates and white rates. Early prenatal care and good nutrition for pregnant women are important elements in giving birth to healthy babies, experts say, and low-income women often do not have access to adequate services.

Over the years Barry has appointed a number of blue-ribbon panels and announced numerous programs to combat the infant mortality problem, but yesterday he stressed that there is no substitute for individual responsibility for the problem.

"Women get pregnant by men, not by government," Barry said. "We're doing all we can to stop every death that's possible. Government by itself cannot solve this problem."

He said he would appoint a special assistant to coordinate efforts to reduce the mortality rate and he has instructed city officials to analyze death certificates to determine the cause of the increase and make recommendations.

Johnson said the mayor's response to the increase was another example of "too much study and too little action" that has characterized the District's approach to combating infant mortality.

The mayor said that federal officials have told him confidentially that progress in reducing infant mortality in other U.S. cities has slowed or stopped. However, no comprehensive figures for 1984 comparing infant mortality rates in the District with those of other cities are available.

The national infant mortality rate improved only slightly in 1984, at a slower rate than in past years, according to figures from the National Center for Health Statistics. The preliminary 1984 rate was 10.6 deaths per 1,000, down from 10.9 in 1983.

The center's figures show that Detroit was the only major city with a higher infant mortality rate than the District's in 1982, the last year for which the center has comparative data.

Barry pointed out yesterday that infant mortality rates among blacks typically are about double those among whites, in trying to explain the sharp rate increase in the predominantly black District. But the 24 deaths per 1,000 black births here in 1984 was higher than the latest national infant mortality rate for blacks of 19.6 per thousand, recorded in 1982.