Relentless downpours last week inundated vast stretches of Bangladesh's northeast, with troops deployed to evacuate households cut off from neighbouring communities.
"The situation caused by flash floods in northeastern Bangladesh has deteriorated rapidly over the last week," Sheldon Yett, UNICEF's representative to Bangladesh, told reporters in Geneva via video-link.
"3.5 million children are in urgent need of safe drinking water. That is a staggering two million more children than just a few days ago. Huge areas are completely under water and are disconnected from safe drinking water and food supplies. Children need help right now."
He said more than 40,000 water points and almost 50,000 toilets had been damaged, warning that water-borne disease can spread very quickly when people are forced to drink contaminated water.
Yett said cases of diarrhoea and other deadly diseases were rising steadily.
"Nearly half a million people have been evacuated into crowded evacuation centres that are not equipped to cater for the safety of women, girls and children," he said.
Ninety percent of health facilities in Sylhet division around the regional capital have been inundated, while more than 5,000 schools and learning centres are submerged, he added.
Yett said UNICEF had brought supplies in by truck as Sylhet airport -- the country's third-largest -- was closed down until Thursday.
The agency has delivered 1,750,000 water purification tablets and 9,000 jerry can water containers, along with thousands of hygiene kits for women and adolescent girls.
"UNICEF is helping children who were lost in the chaos to reunite with their families," said Yett.
Floods are a regular menace to millions of people in low-lying Bangladesh, but experts say climate change is increasing their frequency, ferocity and unpredictability.