NEW YORK - American children's lives are imperiled by health threats ranging from a growing lack of physical fitness to a potential explosion in AIDS cases and child murders, health experts say.

Children are fatter, weaker, more prone to disease and more likely than in the mid-1980s to die from murder or AIDS, according to a study released yesterday.

"With the state of health care today for children, you're sitting on a powder keg," said Dr. Robert J. Ruben, a childhood-disease specialist at Albert Einstein College of Medicine in New York.

The American Health Foundation report card gave an overall C-minus rating to the state of U.S. children's health, citing increases in measles and other childhood diseases and a trend toward obesity at a younger age.

"Our children are fatter, slower and weaker," said Dr. Nicholas DiNubile, a special adviser to the President's Council on Physical Fitness.

DiNubile said studies show half of girls and 25 percent of boys cannot do one pull-up, and 40 percent of children ages 5 to 8 have at least one coronary-disease risk factor.

The report also noted the rates of AIDS cases and murders of children have risen sharply since 1985.

The non-profit research and education group gave higher grades - but no A's - for declines in dental problems and smoking.

Ruben and other participants in a symposium where the report was presented blamed the worsening health levels on rising poverty, the fact that children are less active and lack of extensive school-based health programs.

Their report, based on statistics from the Centers for Disease Control and other sources, said rates for some major childhood diseases dipped in the mid-1980s but have since risen.

Measles cases fell from 13,500 cases in 1980 to 2,822 cases in 1985, but jumped to 9,411 cases last year, the report said. Mumps and rubella showed similar patterns, it said.

The report said black babies were twice as likely to die as white newborns. It said fewer than half of urban children have been vaccinated against measles, mumps, rubella, diphtheria, whooping cough, tetanus and polio by age 2.

Infant mortality also has increased since the mid-1980s. The report showed 9.1 of every 1,000 American babies died in infancy in 1990, up from eight per 1,000 births in 1984 but less than the 12.5 deaths per 1,000 births in 1980, the report said.

Acquired immune deficiency syndrome cases in children under 13 years old jumped from 133 in 1985 to 782 in 1990, the report said. It said the murder rate for youths ages 10 to 19 rose from 1,350 in 1984 to 2,771 in 1989.

However, drug and alcohol use appeared to be declining. The report said the percentage of high-school students who used cocaine in the previous 12 months dropped from 13 percent in 1985 to 3.5 percent last year. Marijuana use fell from 41 percent to 24 percent in the same period, the report said.