FRANKFURT (Reuters) -A planned 3-billion-euro ($3.3 billion) site Thyssenkrupp is building in Duisburg to make carbon-neutral steel could be stranded unless Germany ensures there's sufficient green hydrogen to supply it, the conglomerate's CEO said.

"With this project we are not only pushing the boundaries of what is technologically feasible. We are also currently operating at the limits of economic viability. Or, as things stand today: beyond it," Miguel Lopez said.

The comments cast doubt on what is the company's single biggest investment and reflects a more sobering assessment around hydrogen supply in Europe's top economy, once seen as a viable energy alternative.

Lopez said when the site was conceived the expectation was that sufficient amounts of affordable green hydrogen would be available at the time of completion, assumptions he said had been too ambitious.

"Under the current conditions, there is no guarantee that we will be able to operate the plant economically in the foreseeable future," Lopez said, according to a transcript of a speech held in the regional parliament of North Rhine-Westphalia, where the company is based.

"If this does not change, there is a risk that Duisburg will be home to one of the world's most modern steel production plants - without an adequate supply of the desired green hydrogen."

Lopez confirmed the group was aiming to sell a further 30% stake in its steel division, Thyssenkrupp Steel Europe, to EP Group, which is owned by Daniel Kretinsky and last year bought an initial 20% stake in the business.

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