The government's top health officer said yesterday that AIDS, a deadly new immune system disease striking the country in epidemic proportions, is the Public Health Service's "No. 1 priority."

In his first new conference on Acquired Immune Deficiency Syndrome, Dr. Edward N. Brandt Jr. sought both to dispel growing fears about the danger of contracting the puzzling illness and to combat criticism that the administration failed to respond to it quickly enough.

"There is no evidence to date that indicates AIDS is spread by casual contact," said Brandt, the Department of Health and Human Services' assistant secretary for health. He said the public should not be either "unduly frightened or overly complacent" about the disease.

The cause of AIDS has not been determined, and it has no known cure. Of its 1,450 reported victims, 558 have died since the disease was first reported in June, 1981.

It continues to strike largely in four high-risk groups. Seven of ten of its victims have been homosexual or bisexual males with multiple sexual partners. Most of the others have been intravenous drug abusers, recent Haitian immigrants or persons with the blood disease hemophilia.

But cases also have been reported among other blood transfusion recipients, members of AIDS victims' families, including children, and female sexual partners of drug abusers. Six percent of the victims have no apparent risk factors.

"I personally do no think there is any reason for panic in the general population, Brandt said."

He added that there have been no cases of suspected transmission of AIDS from patients to health workers or from laboratory specimens to laboratory workers.

"On the contrary, our findings indicate that AIDS is spread almost entirely through sexual contact, through the sharing of needles by drug abusers, and, less commonly, through blood and/or blood products," he said. "If I had to have a blood transfusion, I would take one."

Brandt said the government is engaged in "nonstop pursuit to identify the cause of AIDS so that effective treatment and prevention measures can be developed and put in place." He defended the federal effort, saying that $14.5 million will be spent in fiscal 1983, almost the amount spent in eight years on Legionnaire's Disease and nearly three times last year's AIDS expenditures.

Gay activist groups and such congressional supporters as Rep. Henry A. Waxman (D-Calif.), chairman of the House Energy and Commerce health subcommittee, have complained that the government responded too slowly because the victimes largely are homosexual males.

"The administration's response to AIDS has been too little and all but too late. . . . The administration has never asked Congress for money for AIDS and, in fact, has opposed congressional efforts to provide funds to the Centers for Disease Control and the National Institutes of Health," Waxman said yesterday.

But Brandt said the government's response "had nothing to do with members of any high-risk group involved. . . . These people are victims of an illness, and we're going to do everything we can to stop this problem." Brandt announced several new AIDS efforts, including Food and Drug Administration approval of a heat treatment to counter infectious agents in the blood product received by hemophiliacs.

Researchers now say they believe than a infectious organism such as a virus is responsible for AIDS.

New AIDS research grants of more than $2 million are being awarded by the NIH, and the CDC is assigning advisers to cities where AIDS occurs most frequently, including San Francisco, Los Angeles and Miami.