U.S. President Joe Biden blocked Nippon Steel's proposed $14.9 billion purchase of U.S. Steel citing national security concerns, in a potentially fatal blow to the deal after a yearlong review.

Biden, President-elect Donald Trump and an influential labor union opposed the effort by Japan's top steelmaker to acquire the iconic American firm, which would have created the world's third-largest steelmaker, according to World Steel Association data.

The path forward is unclear. The companies could sue the U.S. government, another buyer could swoop in for U.S. Steel, or Republicans who favor the deal could urge Trump to find a way to approve it.