KUALA BERANG: Flight and rail services in India's capital city of New Delhi were disrupted on Tuesday morning as a thick layer of fog enveloped the region, reducing visibility to barely 50 metres (164 feet) in some areas.

"Very dense fog" was observed in the city, where the temperature was just 9 degrees Celsius (48 degrees Fahrenheit), India's weather department said, adding "dense to very dense fog" was likely in the early hours of the day in some areas until Wednesday.

"Dense fog" contains particulate matter and other pollutants, according to the weather department, which warned of the impact this may have on the health of the city's 20 million residents.

New Delhi's air quality index stood at 376 on Tuesday morning, rated as "very poor", according to the Central Pollution Control Board's website. Levels of 0 to 50 are considered to be good.

At least 30 flights that were to land at or depart from the Delhi airport, including on international routes, experienced delays because of the dense fog, according to Indian news agency ANI, in which Reuters has a minority stake.

About 14 trains travelling along routes in North India were similarly affected, ANI reported.

"While landings and takeoffs continue at Delhi Airport, flights that are not CAT III compliant may get affected," the Indira Gandhi International airport said on social media platform X, formerly known as Twitter.

CAT III is a navigation system that enables aircraft to land even when the visibility is low.

Airlines like India's SpiceJet SPJT.NS also took to social media to warn passengers that departures and arrivals from New Delhi may get affected "due to bad weather".

Other parts of North India, including the states of Punjab and Uttar Pradesh, reported similar weather conditions on Tuesday morning, with temperatures ranging between 6 C and 10 C (43 F to 50 F) in most places, the weather department said.