An estimated 400,000 infants have so far been stricken by AIDS and millions will have developed and died from the disease by the year 2000, the World Health Organization said Tuesday.

The U.N. health agency called the situation a "grim social tragedy" caused by mothers infecting their children with the human immunodeficiency virus, which causes AIDS.Transmission of the virus is now mainly heterosexual, which means HIV infection "is rising steeply" among women of childbearing age, the agency said.

"This means a corresponding increase in the number of infants acquiring HIV infection from their mother before, during or shortly after birth," it said.

About 700,000 HIV-infected infants have been born worldwide, mostly in African and other developing countries since the AIDS pandemic began in 1980, WHO said.