IMD officials said satellite imagery showed a dense fog cover over Punjab, Haryana, Delhi, Uttar Pradesh, northeast Madhya Pradesh and Bihar

At 5.30 am, visibility was recorded at zero metres in Agra, Bareilly, Saharanpur and Gorakhpur in Uttar Pradesh; Ambala, Amritsar, Bathinda, Ludhiana and Adampur in Punjab; Safdarjung in Delhi; Ambala in Haryana; Gwalior in Madhya Pradesh; Bhagalpur in Bihar; and Daltonganj in Jharkhand

The IMD issued a red alert for Delhi, warning that fog may disrupt operations at some airports and affect highways and railway routes

An orange alert was issued for several districts in Uttar Pradesh, including Agra, Aligarh, Baghpat, Bareilly, Bijnor, Bulandshahr, Etah, Etawah, Firozabad, Gautam Buddha Nagar, Ghaziabad, Hapur, Hathras, Mathura, Meerut, Moradabad, Muzaffarnagar, Pilibhit, Rampur, Saharanpur, Shahjahanpur and Shamli

Haridwar and Udham Singh Nagar in Uttarakhand and Amritsar, Fatehgarh Sahib, Gurdaspur, Patiala and Sangrur in Punjab are also under an orange alert

The IMD warned of difficult driving conditions and an increased risk of road accidents, adding that there is a possibility of power line tripping in affected areas

"Residents in the affected regions are advised to stay alert and take necessary precautions, including being careful while driving or travelling by any mode of transport, using fog lights, checking road and traffic conditions, staying in touch with airlines, railways and state transport authorities for travel schedules, avoiding unnecessary travel and following advisories issued by concerned agencies," the IMD said

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