WASHINGTON (Reuters) - U.S. Steel Corp, the $14.9 billion acquisition target of Japan's Nippon Steel ( NISTF ), must remain a domestically owned and operated American firm, President Joe Biden will say in a statement on Thursday opposing the proposed merger.

The United States needs to "maintain strong American steel companies powered by American steel workers," Biden will say in the statement, a copy of which was viewed by Reuters.

"U.S. Steel has been an iconic American steel company for more than a century, and it is vital for it to remain an American steel company that is domestically owned and operated," Biden will say.

U.S. Steel shares were down 4.9% in premarket trading on Thursday.

Reuters, citing a source, reported on Wednesday that Biden had plans to express concern over the proposed deal. The issue has the potential to overshadow an April 10 summit between Biden and Japanese Prime Minister Fumio Kishida aimed at boosting the long-standing security alliance between their countries in the face of growing Chinese strength.

Nippon Steel ( NISTF ) clinched a deal to buy the 122-year-old American steelmaker for a hefty premium in December, betting that U.S. Steel would benefit from the spending and tax incentives in Biden's infrastructure bill.

However, several Democratic and Republican U.S. senators have criticized the deal, citing national security concerns or raising questions about why the two companies did not consult U.S. Steel's main union ahead of the announcement.