Residents in India's northern states woke up to another day of poor air quality on Tuesday, as a layer of dense fog shrouded most of the region, and pollution in the capital Delhi remained severe.

India's capital city of Delhi has virtually shut down as air pollution hit "severe plus" levels, according to Indian government officials in its pollution control authority.

Toxic smog engulfed much of northern India on Monday and pollution readings in New Delhi hit their highest of this pre-winter season.

India's pollution control authority said the national capital territory's 24-hour air quality index (AQI) reading was at 484, classified as "severe plus", the highest this year.