Gareth Owen, emergencies director of Save the Children in Britain, said the logistical challenges of reaching tens of thousands of people are hampering all relief efforts.
Save the Children estimates 2.4 million children less than 5 years of age are still to be reached with desperately needed food aid.
Filthy floodwaters still cover approximately a fifth of Pakistan while in some areas villages have become islands with residents having to wade through high waters to reach food distribution centers.
"We are working as hard as we can and as fast as we can to reach children in desperate need of food, shelter and medical care," Owen said. "But as the waters recede, the numbers of families in need of urgent support are only growing."
Owen said he expects the crisis to get "dramatically worse" before it can begin to improve.