Heavy fog delayed dozens of flights and stranded about 9,000 passengers at one of northwest China's Xinjiang Uygur Autonomous Region's main airports on Thursday, airport authorities said.

At least 50 flights were grounded or unable to land due to poor runway visibility, Urumqi International Airport's operational center said in a statement.

The center issued a "yellow" alarm at 10:15 a.m. warning passengers of flight delays, though the airport was virtually unable to operate by 7 a.m.

As of midday, only two flights managed to take off -- a regional jet to Xinjiang's Hotan Prefecture at 11:15 a.m., and a flight to Changsha, capital of central China's Hunan Province, departing at 11:17 a.m. -- both operated by China Southern Airlines.

The local weather bureau forecasts the fog to persist until 2 p.m.