Visibility was low in the city on Monday morning as a layer of smog hung over the national capital with the air quality settling in the severe category.

The city also recorded the season's first dense fog lowering visibility to 150 metres and affecting road and rail traffic movement.

Around 20 trains have been delayed by 15 minute to 2 hours, an official said.

"Fog affecting train movement today. We have taken precautionary measures. Speed restrictions are put in place in such conditions. Safety is our priority. Passengers are informed about schedules through announcements and other means," a northern railway spokesperson said.

However, an airport official said flight operations were not affected as Low Visibility Procedures (LVP) were in place for around four hours starting at midnight.

Satellite images showed a thick layer of fog extending from Punjab to east Uttar Pradesh across Haryana and Delhi, the India Meteorological Department said.

Visibility dropped to 150-200 metres at the Palam Airport between 3:30 am and 6 am and improved to 350 metres by 7 am, an IMD official said.

The Met office reported visibility levels of 25 to 50 metres in Amritsar, Patiala, Bareilly, Lucknow and Bahraich early morning.

The IMD has predicted dense to very dense fog over the plains in northwest India during the next five days.

According to the IMD, very dense fog is when visibility is between 0 and 50 metres, 51 and 200 is dense, 201 and 500 moderate, and 501 and 1,000 shallow.

The Safdarjung Observatory, the primary weather station of the capital, recorded a minimum temperature of 7.2 degrees Celsius. The maximum temperature is likely to settle around 24 degrees Celsius.

Pollution levels were high in the national capital with the air quality index (AQI) settling at 409 at 11 am.

An AQI between zero and 50 is considered good, 51 and 100 satisfactory, 101 and 200 moderate, 201 and 300 poor, 301 and 400 very poor and 401 and 500 severe.