Heavy monsoon rains in India have caused flooding in the country's main sugar producing state Uttar Pradesh, but the full extent of any damage to the crop will not be known until floodwaters recede.

There were apprehensions that a slow start to India's monsoon season would trim cane output in the world's second biggest sugar producing nation, but a late revival in rains resulted in higher acreage being planted. However, fresh floods in North India have now raised fears of damage to the cane crop.

"Sugarcane is at high risk due to floods as water logging can damage the crop," Sudhir Panwar, president of farmers' body Kisan Jagriti Manch said by telephone from Lucknow, the capital of Uttar Pradesh.

According to the latest assessment of the Indian Sugar Mills Association, the country's sugar output could rise 4 percent to 25.3 million tonnes in 2014/15, the fifth surplus year in a row, because of higher cane yields in other major producing states of Maharashtra and Karnataka.

Heavy monsoon rains since Friday in the northern hill state of Uttarakhand caused two major rivers in downstream Uttar Pradesh to rise above danger marks.

"Water is flowing above the red zone on the Ghagra river at Barabanki and the Rapti river at Balarampur of UP," an official at the National Disaster Management Authority told Reuters on Sunday. The floodwaters have affected eight districts of Uttar Pradesh and displaced thousands of people

"Nearly two hundred villages have been hit by the flood which has taken at least 21 lives," said Alok Ranjan, the top bureaucrat of Uttar Pradesh.

India's weather office has predicted more heavy rains in northern hill states, potentially worsening the floods in Uttarakhand and Uttar Pradesh.

India's annual rains have revived from early August after the worst start in five years for the June-September monsoon season, easing the fear of widespread drought in the sub-continent.

But the latest rains have caused landslides and floods in many parts of India and neighbouring Nepal.

In Nepal, flash floods and mudslides following heavy rains killed at least 90 people since Thursday and cut off remote communities in the mountains, according to the latest government update.