At least 26 people have died and nearly 200,000 have been evacuated as a result of massive flooding in the eastern Indian state of Orissa, a minister said on Thursday.

The floods were triggered by torrential monsoon rains across Orissa, causing water levels to breach river banks, prompting a huge rescue operation in which helicopters dropped off emergency supplies to help the stranded.

"The floods have killed 26 people and 12 others are missing," Orissa's Disaster Management Minister Surya Narayan Patra told AFP in the state capital Bhubaneswar, painting a picture of widespread devastation.

"We have evacuated 193,000 people to relative safety," he added, warning that the death toll was likely to rise.

He added that a total of 2.1 million people in the state of 40 million people had been affected in some way or another with floodwaters entering their homes or their crops submerged.

Thousands of people perched on highways and on rooftops to escape the floods -- an annual feature in the coastal state which is also prone to cyclones and tidal surges.

Nineteen of impoverished Orissa's 30 districts have been declared "flood affected," Patra said, adding that nearly 4,100 villages were "waterlogged" while access to nearly 1,300 of them was completely cut off.

Orissa's special relief commissioner Pradeep Kumar Mohapatra told reporters that some 300 relief centres were working round the clock to help the flood afflicted areas.

Helicopters were dropping food and other essential supplies to those trapped by the water in Orissa, he said.

Floods in neighbouring Pakistan have killed at least 270 people and left 200,000 homeless, according to the latest official count.