* Men's biathlon mass start postponed to Tuesday
* Snowboard cross competition also delayed (Updates with women's biathlon mass start to go ahead)
SOCHI, Russia, Feb 17 (Reuters) - Organisers of the Sochi Olympics defied the odds when they battled unusually warm temperatures for a week but they were helpless against a winter fog that caused events to be postponed on Monday.
The men's biathlon 15km mass start was called off for a second straight day and thick fog also forced the postponement of the men's snowboard cross competition.
The women's biathlon 12.5km mass start, however, did get the all-clear for later on Monday.
"I think it is actually quite ironic that the biggest issue we've had so far is due to winter fog and that's led to the biggest postponement we've had," said IOC spokesman Mark Adams.
The men's biathlon event has been set for Tuesday. Seeding runs for the men's snowboard cross were initially scrapped on Monday, with organisers hoping to go straight into the finals, before postponing those as well.
The morning's light drizzle also affected the conditions For biathlon, with the snow being extremely slushy.
"The salt on the track isn't working. They salted the whole course but there seems to be no interaction," said competition jury member Patrizio Curtaz.
With memories of Vancouver's melting venues in the first few days of the 2010 Games still fresh, temperatures in Sochi rose to over 18 degrees Celsius during the first week.
Russian organisers answered the challenge with batteries of snow cannons, careful planning and chemicals to keep the pistes and courses up to Olympic par.
"The number of snow cannons concentrated on one single resort is unique," said Rosa Khutor resort representative Sergey Belikov on Monday.
"We cover a zone of about 100 hectares, which is a huge area. I would like to assure all of you that there is no risk to the quality of the surface and there is no risk that the snow will melt at the ski resort."
Backup snow, stored from last year, was not needed for the high-speed events, he said.
"If we talk about high-speed competitions, we don't host those on snow, we don't host those on a mixture of artificial and natural snow, those are hosted on ice," Belikov said.
"We managed to create a cushion of snow - a mixture of natural and artificial snow - that is more than a metre and a half thick, and on top of it we have water. So if this holds, this will stay in place for a long time."
The Russians have made good on their promise in 2007 to guarantee snow even if athletes raced in short sleeves or tank tops due to the heat.
But if the fog does not clear up over the Caucasus mountains more postponements could hit the Games. (Additional reporting by Julien Pretot in Rosa Khutor, Reporting by Karolos Grohmann; editing by Robert Woodward)