After remaining weak for a couple of days, foggy conditions strengthened in the national capital on Thursday, causing flight delays even as the minimum temperature dipped to 5.6 degrees Celsius.

The fog remained feeble till around 6am but after that it strengthened and enveloped the city, reducing visibility to 100 metres, the Indian Meteorological Department said.

"Up to 0600 IST, it was all okay at the runway, when the RVR (Runway Visibility Range) was recorded at 1100 metres. But by 0615 IST, it reduced to 300 metres and went on decreasing. At around 0800 hours, the RVR was mere 150 metres," an Airports Authority of India spokesman said.

Some morning flights had already taken off by the time the fog started gathering. However, after 0630 IST, flights could not take off and air services from the capital could resume only after 1000 IST. An international flight -- Lufthansa 762 � landed using the CAT-3A landing system.

• Fog hits train movement, Delhi flights on schedule

Train services continued to be disrupted due to foggy conditions in north India, with as many as six trains cancelled and many delayed by several hours.

Among cancelled trains were Kalindi Express from Delhi to Farukhabad, Unchahar Express from Ambala to Allahabad and Lal Qila Express from Delhi to Sealdah.

Late running trains included Mahananda Express from Katihar to Delhi, which was late by 18 hours, a railways spokesman said.

Meanwhile, the minimum dropped from Wednesday's 7.7 degrees Celsius to 5.6 degrees Celsius, one degree below normal. The maximum temperature dropped to 19.3 degrees Celsius, two below normal, the IMD said.