The Centre said it was monitoring the situation at the Indira Gandhi International Airport where flight operations were affected this week because of heavy fog, PTI reported. Nearly 500 flights, including domestic and international, have been delayed or cancelled since Sunday.

“Really sorry for all the delays and inconvenience caused”, Minister of State for Civil Aviation Jayant Sinha said in a tweet. “We have been continuously monitoring the situation and trying to improve the system performance. All suggestions welcome.”

Sinha’s tweet came after the Chief Strategy and Commercial officer of Tata-SIA run Vistara airline Sanjiv Kapoor brought up the “terrible delays” at the Delhi airport because of fog. “Entire system thrown out of whack,” Kapoor tweeted. “We allowed free changes and cancels for yesterday and also today.”

Dense fog has affected visibility in Delhi, Haryana, Punjab and Uttar Pradesh since Monday. Train and flight services in the national Capital continued to suffer for the third straight day. On Wednesday, over 59 trains were delayed, 13 rescheduled and 21 cancelled, NDTV reported.

The weather department has predicted that the air quality in the national Capital will improve from Wednesday, however, they said it will continue to remain “very poor”.

“The pollution level in the Capital which was mostly in “very poor” category for the past two weeks is now at “severe” category only until January 2, 2018. It will slip back to “very poor” from January 3 onwards,” the System of Air Quality and Weather Forecasting And Research, or Safar, said on Tuesday.