The homeless and the elderly are the worst affected
Dozens of people have died in a cold wave sweeping through northern India, the country's state broadcaster Doordarshan says.
The majority of deaths have happened in the state of Uttar Pradesh, according to media reports.
Most deaths so far have taken place among the homeless and elderly, and local authorities have been asked to arrange shelter for the vulnerable.
Scores die in India every year, being ill-equipped to deal with extreme cold.
Estimates of the number of dead vary from 25 to 100 but these figures cannot be confirmed at present.
Fog in central Punjab region in neighbouring Pakistan has also shut down highways and affected railway and flight schedules.
A number of people have been injured in some minor accidents due to fog on Monday morning, the BBC's M Ilyas Khan says.
Heavy fog and a cold wave have disrupted life across northern India with temperatures dropping to zero degree Celsius in several places, including the city of Amritsar in Punjab.
Punjab, Haryana and Uttar Pradesh are among the northern states which have been hit by intensely cold weather.
In Uttar Pradesh, scores of homeless people have died after being exposed to the intense cold.
The fog caused two separate train accidents
The victims were mostly poor people who were sleeping on the streets or out in the open.
There are few homeless shelters in Indian cities and towns and although the authorities have distributed blankets and firewood, their efforts have been inadequate in the face of the extreme cold, says the BBC's Sanjoy Majumder in Delhi.
Poor visibility because of dense fog has also affected rail and air traffic in the region with several flights and trains cancelled, leaving tens of thousands of passengers stranded.
On Saturday, the fog caused two separate train accidents in Uttar Pradesh leaving 10 people dead and nearly 50 injured.
Railway officials said the impact was minimal as the trains were travelling at well below their normal speed because of the dense fog.
Weather officials say the temperatures are expected to stay low over the next few days.