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, after dense fog overnight.

Delhi recorded air pollution more than 17 times above an acceptable level as worsening smog triggered curbs on trucking and shuttered construction sites and schools.

The smog, a toxic blend of smoke and fog, happens each year in winter as cold air traps dust, emissions, and smoke from illegal farm fires in some surrounding states. Visibility dropped to 100 m (109 yards) in Delhi and Chandigarh,