The season's first dense fog blanketed Delhi and neighboring areas on Monday morning, lowering visibility to 150 metres and affecting road and rail traffic movement.

The India Meteorological Department said the Indo-Gangetic plains are likely to see "dense to very dense" fog over the next five days, which may result in cancellation and diversion of trains and flights.

A railway official said around 20 trains were delayed by 15 minutes to 2 hours in the morning.

"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 IMD 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 Safdarjung Observatory, the primary weather station of the national capital, recorded a minimum temperature of 7.2 degrees Celsius.

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

Dense to very dense fog is predicted over the plains in northwest India during the next five days.

"Due to moisture and light winds at lower tropospheric levels over Indo-Gangetic plains, dense to very dense fog in many/some pockets very likely over Punjab, Haryana, Chandigarh, Delhi, Himachal Pradesh and Uttar Pradesh in night/morning hours during the next three days and dense fog in isolated pockets for subsequent two days," the Met office said in a statement.

The IMD also issued an advisory, saying difficult driving conditions on highways, some collisions and tripping of power lines are also possible.

"Likely train delays, diversons and cancellations. Airport operations are likely to be affected with flight delays and cancellations," it said.

The weather department said long exposures to fog can cause wheezing, coughing and shortness of breath in people having asthma bronchitis. It could also lead to eye irritation or infection.

The Met office suggested that travellers carry essential items like water and medicine on long journeys.

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