The District of Columbia's infant mortality rate, already one of the highest in the nation, rose 9.6 percent last year while at the same time the national rate dropped 11.9 percent, city officials said yesterday.

In 1976, the city had the highest infant mortality rate of 26 cities with populations of more than 500,000, when ranked together by the federal government for comparison purposes.

In 1977, according to preliminary figures released yesterday by the city's Department of Human Resources, the city's infant mortality rate was 27.3 deaths for every 1,000 births. The mortality rate for 1976 was 24.9. Nationally, according to the DHR figures, the rate was 14.1, while in 1976 the nationwide infant mortality rate was 16 per 1,0000.

Dr. Raymond Standard, chief of the city's Community Health and Hospitals Administration, said yesterday there is an indication that the increase in deaths was due to birth defects.

It is unlikely, however, that the entire increase in deaths locally can be attributed to birth defects, as the increase in the mortality rate translates roughly to 30 additional deaths last year, too large a number to be accounted for by a single cause.

While various aberrations cause annual fluctuations in the infant mortality rate in all cities, the District has been consistently high for the past decade.

From 1970 through 1976, the city ranked number one in the country in infant mortality for five years, and took third place in 1973 and second in 1974.

Washington's rate was higher in 1976 than such cities as Detroit, Cleveland, Philadelphia, Boston, Chicago, New York City, Los Angeles, Memphis, Pittsburgh and New Orleans.

A news release issued yesterday through the public affairs office of the D.C. government cited the preliminary figures as part of an announcement of a city program designed to reduce infant mortality.

Albert P. Russo, director of DHR, said he himself had not seen the figures or any explanation for them and could not comment on them.

The program to which the release referred was the federally funded Improved Pregnancy Outcome project, launched too late in 1977 to affect that year's infant mortality rate.

According to the release, the city has placed "outreach workers in five District schools to counsel adolescents on the need for proper prenatal care." In the past month the workers have counseled approximately 45 pregnant students, the department said.

More than 2,000 D.C. teen-agers gave birth in 1976, according to federal statistics.

However, at the same time Washington health officials have placed the initial thrust of the IPO project on reducing teen-age births, federal statistics indicate those births are already declining dramatically.

According to statistics released last week, there was a 19 percent drop in teen-age births in the city between 1974 and 1976, apparently because of an increase in abortions among teen-agers.

Additionally, a study by Dr. Armand Pulcinella of George Washington University Medical Center shows that only 34 deaths, or 15 percent of the infant deaths in 1976, occurred among babies born to teen-agers.

Thus the project's stated goal of reducing by 10 percent the infant mortality among the babies of teen-agers would only cause a decline of 1.5 percent in the number of infant deaths.

City officials defend the project's goals by saying that it is only beginning with teen-age pregnancy, and will focus on other groups of mothers during the $2 million, five-year, life of the effort. Teen-age pregnancy, these officials point out, resulted in about a quarter of the city's births in 1977.

Dr. Gordon Avery, chief of neonatology -- the care of newborns -- at Children's Hospital National Medical Center, said yesterday the continued rise of infant mortality in the District, when mortality is going down nationally, "is certainly a big problem, distressing, and needing everybody's attention."

Attacking teen-age pregnancy is "perfectly fine," said Avery, "but even if you wiped (it) out you wouldn't wipe out infant mortality. It's certainly on the right track, but not sufficient."