BEIJING, May 16 (Reuters) - China warned on Friday that Beijing will break its promise to provide athletes with clean air during the Olympics unless polluting industries step in line and cut emissions.

Beijing's pollution has already proved a major concern for athletes, with twice Olympic champion Haile Gebrselassie, who suffers from asthma, pulling out of the men's marathon out of concern for his health.

In addition to shutting down high polluters within city limits, Beijing has demanded five surrounding provinces scale back or stop production to ensure blue skies hang over Olympic venues for the Games in August.

But last month, a top iron and steel association official said that steel mills were holding out for direct subsidies in return for scaling back production, indicating reluctance at local levels to wear economic losses to serve national interests.

In an unusually frank departure from Olympic organisers' optimism, Environmental Protection Minister Zhou Shengxian warned that Beijing would be at the mercy of nature to guarantee clean air at the Games.

"At the current level of progress and intensity, we can meet national standards and our government's Olympic pledge under ordinary weather conditions," Zhou said in a speech to top government and industry officials carried in the ministry's official newspaper on Friday.

"But we must be clear-headed that the task at hand is still challenging, and the environmental situation is still quite grim."

Zhou described the promised installation of clean technologies across the regions as "very unbalanced".

"In all localities, there are still obvious problems regarding these projects' slow or inconsistent deployment," Zhou said. "We must pay great attention to these and not drop our guard."

Beijing has yet to make public detailed shut-down orders or name factories required to cut back production but has flagged harsh measures on industries if smog fails to clear.

Zhou demanded a crackdown on recalcitrant polluters and said thermal power plants that failed to install promised sulphur-scrubbing technology should be curtailed or stopped from generating power during the Games.

He suggested the fight to ensure clean air could be carried right into the Olympic period.

"As soon as there is any unfavourable weather during the Olympic period, surrounding regions must take serious measures to cooperate in delivering clean air targets," Zhou said. (Reporting by Ian Ransom; Editing by Nick Macfie)

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