BBC Correspondent in Baghdad
The head of the UN Children's Fund, Carol Bellamy, says a sharp rise in acute diarrhoea is hitting children in Iraq already weakened by malnutrition.
Iraqi children face uncertain dangers
She is in the country to assess post-war conditions and to hand out school supplies as part of a Unicef push to get schools up and running as soon as possible.
Ms Bellamy said war-related damage was aggravating poor health conditions that existed before the conflict.
The breakdown in sewage and water treatment plants was the main problem, she said.
They had already been poorly maintained because of UN sanctions.
Many stopped working altogether during the war and the looting that followed.
One quarter of Iraqi children were already malnourished before the bombs started dropping and doctors said that, with poor sanitation, the situation was getting worse.
Ms Bellamy also stressed the need to get children back into school as soon as possible, even if conditions were not ideal.
Threat of violence
Besides the damaged buildings and a shortage of supplies, Ms Bellamy believes that many parents are afraid.
But she says this may be more because of rumours of danger than actual threats.
"Whether it is accurate or not, there appears to be a perception on the part of many families that the children, particularly the youngest children, are potentially subject to violence if they come to school.
"We will work to try and dispel that," she said
Ms Bellamy believed there was much work to do with street children but could not say how many there were.
According to Unicef officials, it was difficult to deal with the problem before the war because the previous Iraqi government refused to acknowledge it.