The nation's unemployment rate held at 9 1/2 percent in August, as the number of people looking for work increased by more than the number of new jobs, the government reported yesterday.

Despite the levelling off in the unemployment rate, the new figures showed that the "labor market continued to improve in August," according to Bureau of Labor Statistics Commissioner Janet L. Norwood. Almost 300,000 jobs were created last month, and factory overtime increased, she said.

The rise in jobs did not show up in an improvement in the unemployment rate, because it was more than matched by an increase in the number of people looking for work, the Labor report said. The civilian labor force climbed by 404,000 in August after a slight decline in July. Adult women and teenagers accounted for all of the labor force increase.

Black unemployment, meanwhile, jumped from 19 1/2 percent in July to 20 percent in August, as the number of black teenage boys looking for work soared to a record 56.8 percent, the Labor Department said. "We find there is a large disparity . . . between the whites and blacks," said Norwood. "I think the problem of jobs for our black teenagers is a serious one."

Several analysts had expected unemployment to hold steady or decline only slightly in August after rapid declines in the early months of the recovery. "The general trend is downward, but slowly downward. You've got a combination of growth in the labor force and slower growth in the economy," said Nariman Behravesh of Wharton Econometrics in Philadelphia.

In July the jobless rate dropped sharply from 10 percent to 9 1/2 percent, and it now stands 1.3 percentage points below the 10.8 percent peak reached last December. In August, total civilian employment reached an all-time high of 101.6 million, while unemployment was 10.7 million, after seasonal adjustment.

White House spokesman Larry Speakes said the August "pause in declining unemployment rates comes on the heels of an unusually large drop in July. A downward trend over several months, combined with the increase in payroll employment, point to a steadily improving jobs picture. Economic news continues to be encouraging."

But the economic recovery that roared ahead in the second quarter of the year and has boosted employment by 2.4 percent since last November may now be slowing, analysts say. This would mean that the jobless rate will fall more slowly in coming months than it has done so far during the recovery.

The AFL-CIO said yesterday that there "is no recovery for the one of 10 Americans still out of work."

Blacks as a group so far have not shared in the unemployment improvement since the recession ended, yesterday's report showed. "The overall jobless rate for black workers . . . has shown little change since December," Norwood said. She added, however, that "some improvement is apparent for adults. At 18.4 percent, the jobless rate for adult black men has dropped by about 2 percentage points since December."

Other major groups had little change in their jobless rates between July and August, the Labor report said.

Norwood said that the decline in unemployment since the recession ended last November was much steeper than the drop in the first few months of the 1975 recovery and that this year's rise in employment was somewhat greater than in the early months of the past few economic recoveries. However, the recession that the economy now is climbing out of was one of the worst since World War II, and it pushed unemployment to its highest levels since the Great Depression.

Some industries have recovered much more of the ground lost during the recession than others, Norwood said. She cited the auto, rubber and lumber products industries as among those that have picked up more of the lost jobs, while the steel industry has done less well.

Last month's employment figures were distorted to some extent by strikes among telephone workers and some others, Norwood said. These cut 710,000 people from payrolls in August, lowering total payroll employment by 410,00. After correcting for the effects of the strikes, "The business survey would have shown an increase of close to 300,000 over the month," Norwood said.

"The continued rise of factory hours in August is noteworthy because employers usually expand workers' hours before hiring additional workers," she said. Factory hours edged up from 40.2 hours in July to 40.3 hours last month, while the overall workweek for private industry was unchanged at 35 hours.