Immediate relief must be provided to millions of people by floods in southern Pakistan or more lives will be lost, Oxfam has warned.
The international aid agency said it was launching an emergency aid response to help flood victims in the Sindh province.
It said that some 5.3m people were now affected by the floods, caused by days of torrential rains.
More than 200 people have already been killed by the floods, reports say.
The growing crisis is already being compared with last year's devastating floods.
Millions were displaced across the country and about 2,000 people died as torrential monsoon rains in 2010 caused rivers to burst their banks, washing away homes and property. Sindh was one of the worst affected regions.
"There is an urgent need to provide immediate and life-saving relief to the millions affected," Neva Khan, Oxfam's country director in Pakistan, said on Tuesday.
"It hasn't stopped raining in Sindh for the last 10 days. Large swathes of land are underwater and people are desperately awaiting relief.
"They have lost their crops, homes and livestock for the second time - and been pushed from last year's disaster to this one."
The agency - which is already providing drinking water and sanitation to thousands of people in the area - said the world community must act immediately because "the crisis is multiplying each passing day".
Zafar Qadir, the head of Pakistan's disaster management authority, told AFP news agency that 209 people had so far been killed.
In its appeal, Oxfam also warned that the government's slow repairs of river embankments and other defences damaged in last year's disaster had made the population more vulnerable this time.
On Monday, the UN said it had begun efforts to feed 500,000 people affected by the floods and rain, initially concentrating its efforts in Sindh's badly-hit Badin district.
The US has already send food aid. It is also promising tents, medicine and other non-food items.
China has pledged $4.7m (£3m) for urgent humanitarian assistance.