Rotting corpses contaminating water sources and poor sanitation amid devastating floods in northern India could lead to a serious outbreak of diseases such as cholera and dysentery, aid groups warned on Wednesday.

The floods, triggered by heavy monsoon rains more than 10 days ago, have killed at least 822 people in Uttarakhand and forced tens of thousands from their homes. Officials say the death toll may cross 1,000 and thousands are still reported missing.

Authorities have so far been focusing on rescuing thousands of pilgrims who visit the region for its sacred Hindu temples and shrines, but aid agencies, struggling to get past roads choked by landslides to local villagers, warned of another disaster unfolding in form of an outbreak of diseases.

Aid workers said they were concerned that a combination of heavy rains and corpses lying out in the open would contaminate streams and rivers.

"We are getting reports from the field that there are rotting bodies lying around, many of them semi-buried in soil and rubble that came down from the mountains," said Zubin Zaman, Humanitarian Manager for Oxfam India, which is working in Rudraprayag, one of the worst affected districts.

"There are also carcasses of livestock in rivers and streams and this has, of course, contaminated so many of their water sources. But people are desperate and are being forced to consume water they wouldn't otherwise."

Zaman said he was concerned of outbreaks of water-borne diseases such as cholera, diarrhoea and dysentery, adding that he had received reports that 400 people were admitted to a medical camp in Sonprayag.

The disaster - the worst floods India has witnessed since 2008 when around 500 died in the eastern state of Bihar - has swept away buildings, washed away farmland and destroyed major roads and bridges.

The floods and landslides have been dubbed a "Himalayan tsunami" by the Indian media due to the torrents of water unleashed in the hilly region, which sent mud crashing down, burying homes and other buildings.

Heavy rains over the last two days have hampered rescue operations by the army and air force who have been air lifting survivors marooned in and around the four temple towns of Kedarnath, Badrinath, Gangotri and Yamunotri since June 15.

Television channels broadcast dramatic pictures of desperate pilgrims scrambling to get aboard dozens of military helicopters which have been ferrying people to safety. Around 96,500 people have so far been evacuated by land and air, according to media reports.

An air force rescue helicopter crashed on Tuesday, killing 20 people on board. The air force said the helicopter was delivering wood for the mass cremation of bodies found in and around the temple town of Kedarnath.