Unemployment among nonwhite teen-agers in the District of Columbia reached an official level of 48.5 percent last year, more than four times the jobless rate for the city's white youth, the D.C. Manpower Department reported yesterday.

An expert in minority employment problems, John E. Jacob, president of the Washington Urban League, called the official figure "alarming" and said that it drastically understates an actual jobless rate of close of 70 percent.

Jacob said the official statistics cover only those who apply for jobs through government-sponsored job programs, and not those looking for work on their own or who feel discouraged from looking.

"It is a far more critical problem than we had cared to come to grips with in this community," Jacob said. "There is only one solution, (and) that is to create jobs."

Until the release of the new figure yesterday, officials had said the unemployment rate last year for the city's mostly black teenage population was about 40 percent.

The actual unemployment figure for D.C. youth of all races in the 16-to-19 age group, compiled by the U.S. Department of Labor's Bureau of Labor Statistics, was 41.6 percent. The bureau called this "disturbingly high."

"Among nonwhite 16-to-19-year-olds, the unemployment rate was 48.5 percent, compared to 11.2 percent for white teen-agers," the bureau said.

Job finding problems also are severe for nonwhites in the young adult age group of 20 to 24 the bureau reported. It said the jobless rate was 20 percent for D.C. nonwhites in this group, compared with 5.8 percent for whites. The unemployment figures drop sharply for older workers.

The total number of unemployed youths of all races was about 7,000, the bureau said. The figure, an average for the entire year, represents a combination of summer vacation job-seekers and youths permanently out of school.

The jobless rate for teen-agers was far greater than for the population as a whole, according to other unemployment figures announced yesterday.

In April, the unemployment rate for the entire work force for the Washington metropolitan area was 4.5 percent, seasonally adjusted. For the District of Columbia alone, it was 7.7 percent, the same as for March.

The 7.7 percent figure is only 0.1 percent higher than the average jobless rate for all of 1975, when the local economy began a downturn and unemployment began to climb to a peak of 9.7 percent last year.

"The structure of the Washington labor market, dominated by government and business services employers, has obviously generated relatively less employment opportunities for teens than for adults," the m anpower department said in yesterday's announcement.

The District's 48.5 percent rate for jobless minority youth last year was substantially higher than the nation-wide rate for the same group, which averaged around 40 percent , according to another Bureau of Labor Statistics report.

Unofficially, in New York City, as much as 86 percent of the nonwhite youths lacked jobs last summer.

The Manpower Department's analysis of the new unemployment figures for D.C. included these other findings.

Unemployment rates are lower for women (7.8 percent) than for men (11.3 percent). The situation here is different from that in the rest of the country, because so many women hold clerical jobs with the federal government here.

Nonwhite D.C. residents had much higher rates of unemployment (12.2 percent). For nonwhites, this was an increase from 10.3 percent in 1976.

In April, the new report said, the total number of jobs in the metropolitan area increased by 10,000 to a total of 1,402,100, including 4,700 new jobs in the construction industry and 2,700 in retail trade. Government employment was down slightly.