MULTAN, Pakistan (AP) — Fever, stomach problems and skin diseases are spreading among Pakistani flood victims, officials said Friday, adding another dimension of danger to a crisis that could get even worse, with the U.N. warning that dams in the south may burst.

Aid workers warn that waterborne diseases and other illnesses could raise the death toll from more than two weeks of flooding to well past the estimated 1,500 people who have perished so far.

The U.S. said Friday that it would give $3 million to help establish 15 treatment centers for waterborne illnesses in the aftermath of the floods, which are estimated to have directly or indirectly affected some 14 million people.

In the Multan area of Punjab province, medical workers have seen at least 1,000 children with illnesses such as gastroenteritis in the last three days, said Mumtaz Hussain, a doctor at the main government hospital.

"The situation is alarming as the diseases can infect other survivors," Hussain said. The hospital is treating victims at its main facility but also has set up 12 medical camps in the area.

The floods have been described as the worst natural disaster in independent Pakistan's 63-year history.

Up to a quarter of the country has been affected by the floods, though not all those areas were necessarily under water, U.N. spokesman Maurizio Giuliano said. The hardest-hit region has been the northwest, where farms and infrastructure were badly damaged.

The U.N. warns that the crisis was far from over, saying dams in southern Sindh province could still burst in the coming days as bloated rivers gush through. More rains are expected over the weekend, and monsoon season is forecast to last several more weeks.

U.N. officials have launched an immediate appeal for $460 million in aid for Pakistan. The U.S. has donated more than $70 million and has sent a ship and helicopters to assist relief efforts.

To help stop the spread of illness, the U.S. is distributing hand soap and has provided mobile water treatment units that can provide clean drinking water for 10,000 people a day, the U.S. Embassy said in a statement.