Four of the deaths were in Punjab state and the other three were in Uttar Pradesh, the Press Trust of India (PTI) said quoting government officials.

Flights to Delhi's international airport were cancelled until late morning after fog reduced visibility to around 50 metres (165 feet), delaying at least 60 flights, they said.

The fog also delayed 18 trains and caused numerous road accidents, PTI said.

Winter usually takes a heavy toll around impoverished South Asia. Last year, some 400 people died from cold in Uttar Pradesh alone.