( CNN ) -- As the relief effort in flood-hit Pakistan intensified Monday , officials were bracing themselves for an outbreak of disease among the millions affected by the country 's worst deluge in 80 years . Rushing floodwaters and mudslides triggered by monsoon rains have engulfed large swathes of northwestern Pakistan since Thursday , killing an estimated 1,500 people said Information Minister Mian Ifthikar Hussain on Monday . Now government officials are concerned a lack of drinking water is spreading conditions such as cholera and gastroenteritis in affected areas such as the Swat Valley in Khyber-Pakhtoonkhwa Province , Agence France-Presse reported . `` We estimate that about 100,000 people , mostly children , have been hit by cholera and gastro diseases , '' Syed Zahir Ali Shah , health minister for Khyber-Pakhtoonkhwa , was quoted by AFP as saying . iReport : Are you there ? Send pictures , video . `` Our priority is to first evacuate them to safe areas and then provide them with medical treatment , '' he added . But the rescue and recovery efforts could become more complicated as monsoon rains continued to fall across much of Pakistan Monday . `` Anything they get will aggravate an already devastating situation , '' said Brandon Miller , a meteorologist with CNN International . The Pakistan Meteorological Department said Monday that it expects enhanced monsoon activity for the first two weeks of August . It forecast widespread rain in the next four days in Sindh , Punjab , Kashmir , Khyber-Pakhtoonkhwa , northeast Balochistan and Islamabad . Areas along the Indus River will be badly affected due to extremely high flood conditions , the department has said . Special : Impact Your World . Tens of thousands of people remain trapped on rooftops and in higher areas trying to escape rising floodwaters , according to the United Nations . `` We 've got the government sending boats and helicopters to try to reach people and bring them to safety at the same time as trying to deliver emergency relief , '' said Nicki Bennett , a senior humanitarian affairs officer for the U.N. Damaged roads and bridges have made rescuing stranded residents difficult , she said , noting that even a U.N. warehouse where the organization stores food , blankets , soaps and bucks is partially underwater . The Pakistani military , which has deployed dozens of helicopters and boats across the region , said 25 bridges have been destroyed and some 36 miles ( 58 kilometers ) of road washed away in Swat and Shangla districts . Despite these difficulties , senior commanders in the region told reporters that tens of thousands of people had been rescued from Shangla , Peshawar , Nowshera and Charsadda . In the hardest-hit province , Khyber-Pakhtoonkhwa , authorities estimate that 800 people have died and more than 100 are still missing , the World Health Organization said . `` The actual figures could be higher as communication networks in many areas are still disrupted , '' it said in a report . About 15,000 families in the province need emergency assistance , such as food , clean water , shelter and medical services , the organization said . `` As the water level recedes , measures to prevent water-borne diseases will also become a priority , '' it said . U.N. Secretary-General Ban Ki-moon late Sunday authorized $ 10 million in emergency aid . This follows an earlier pledge by the United States to send helicopters , boats , pre-fabricated bridges and water-filtration units . The European Union and China have also committed to provide humanitarian aid . A Pakistani Red Crescent official told CNN that the number of people affected by the floods has risen to nearly 2.5 million people , with infrastructure receiving major damage . Rushing water also has washed away thousands of acres of crops , government buildings , businesses , schools , bridges and homes , officials said . CNN 's Reza Sayah , Nazneen Akbari and Matiullah Mati contributed to this report .
NEW : 2.5 million people killed in floods said Information Minister Zahir Ali Shah . U.N. says Up to 1,500 people in northwestern Peshawar affected . Officials fear an outbreak of gastro conditions due to lack of drinking water . Destroyed bridges , roads hampering rescue efforts in flood-hit areas .