By Karolos Grohmann

SOCHI, Russia (Reuters) - Organizers 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.

"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 biathlon event has been set for Tuesday. Seeding runs for the men's snowboard cross were initially scrapped on Monday, with Organizers hoping to go straight into the finals, before postponing those as well.

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 Organizers 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 meter 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.

(Reporting by Karolos Grohmann; editing by Robert Woodward)