As many as 576,000 Iraqi children may have died since the end of the Persian Gulf war because of economic sanctions imposed by the Security Council, according to two scientists who surveyed the country for the Food and Agriculture Organization. The study also found steeply rising malnutrition among the young, suggesting that more children will be at risk in the coming years. The results of the survey will appear on Friday in The Lancet, the journal of the British Medical Association.