"They were in school at the time when so many other school buildings came down," she told a news conference in the Pakistani capital Islamabad after visiting the disaster area.

"The ones that survived, many have injuries. The ones that survived, also many lost friends. They lost teachers, they lost important people in their lives."

The government says more than 55,000 people died in Pakistan in the Oct. 8 earthquake while more than 78,000 were injured.

"We know that children under 18 are about half of the population in the affected areas," Veneman said. "And therefore we think that about half of the victims, either injured or the dead, have been children."

She said UNICEF estimated at least 17,000 children were killed in schools destroyed in the quake, which struck during morning classes on a Saturday. "That is one number we have some estimates on," she said.

A massive UN-led relief effort has been struggling to ensure survivors in remote mountains settlements have shelter and sufficient food for the coming Himalayan winter.

Veneman said children who had survived the quake were still under threat. "We are concerned about the possibility of a second wave of loss of life if children don't get the right interventions," she said.

"The right interventions being health care if they are injured, immunisations so that there aren't large scale disease outbreaks ... clean water so that they don't get tremendous number of kids dying from diarrhoea."

She said most hospitals in the quake area had been destroyed or damaged and many medical staff killed or injured, yet UN appeals for emergency funds had yet to be properly answered.

"The appeals that the UN have asked for have gone not completely fulfilled and in fact not even a fraction of the way," she said. "So we think that efforts need to be made by the international community to provide the relief here."