UNICEF said at least 3 million people, more than one-third of whom are children, have been affected by the country's flooding, the worst in 80 years.
Children are the most vulnerable and face a great threat from hunger, cholera and scabies, Dr. Muhammad Rafiq of UNICEF told London's Daily Mirror from Khyber Pakhtunkhwa, the hardest-hit province.
"Our biggest concern is places we haven't been able to reach. It's a disaster that will be felt for years," he said.
One-third of Pakistan's 135 districts have been affected by the flooding.
The World Health Organization said critical health issues include the control of waterborne diseases such as diarrhea and respiratory infections, treating the injured, ensuring the quality of drinking water and enhancing public access to health facilities.
"There is a tremendous need for more medical and related materials to treat people affected by the humanitarian emergency, as well as to immunize children, particularly against polio and measles," WHO said in a news release.
WHO is dispatching medicine and health supplies enough to treat more than 200,000 people, the organization said.
WHO said 15,000 people have been treated so far, many for diarrhea. Medical teams are also focusing on maternal, neo-natal, child health, nutrition and psychological assistance.
But the agency said that at least 39 health facilities were destroyed in the floods, with tons of medicines lost.
The U.N. World Food Program said that just in Khyber Pakhtunkhwa, an estimated 1.8 million people are in need of food assistance.
"We are prioritizing the worst-affected areas," said WFP Executive Director Josette Sheeran in a news release. "More distributions are due to start as WFP mobilizes staff to overcome immense logistical challenges."
The U.N. refugee agency said so far it had delivered 10,000 tents and other relief items in the Khyber Pakhtunkhwa and Balochistan provinces.
The refugee agency said it aims to assist at least 240,000 of the most vulnerable flood victims with shelter supplies, blankets, buckets and other items.
Most of the flood-hit displaced, which include Afghan refugees and Pakistanis who were already displaced, are crammed into schools and other public buildings.
"The Pakistani people of this region have been serving as the generous hosts of more than a million Afghan refugees," said Antonio Guterres, U.N. High Commissioner for Refugees in a statement. "Now is the time for the international community to demonstrate the same kind of solidarity with them."