Activist investor Ancora Holdings has built a stake in U.S. Steel and wants the steelmaker to drop its merger agreement with Japan's Nippon Steel, the Wall Street Journal reported on Sunday, citing sources.

The exact size of Ancora's stake in U.S. Steel could not be determined. The activist investor also intends to rally shareholders around a plan to oust U.S. Steel's top boss David Burritt, the report said.

Ancora is not interested in pursuing a sale of the American steelmaker to another party, the WSJ reported, adding that it has nominated nine director candidates to the company's 12-person board, including Stelco's former chief Alan Kestenbaum.

Ancora, U.S. Steel and Nippon Steel did not immediately respond to Reuters' request for a comment outside regular business hours.

Earlier this month, former U.S. President Joe Biden blocked Nippon Steel's $14.9 billion deal for U.S Steel on national security grounds and delayed an order until June for Nippon to abandon the bid.

The companies have sued the Biden administration for blocking the acquisition.

Donald Trump, who was sworn in as U.S. president on Jan. 20, has also voiced opposition to Nippon Steel acquiring U.S. Steel.

"I will block this deal from happening," Trump wrote on his social-media platform Truth Social in December.

Reuters reported earlier this month that rival steelmaker Cleveland-Cliffs was partnering with peer Nucor for a potential all-cash bid for U.S. Steel.

Cleveland-Cliffs had previously proposed acquiring U.S. Steel, but the American steelmaker raised concerns about potential antitrust issues and the consolidation of steel supply to U.S. automakers. A potential deal could result in up to 95% of U.S. iron ore production being controlled by a single company.