(Reuters) -The United Steelworkers union expressed concerns about Nippon Steel's latest proposal that gives the U.S. government veto power over any potential reductions in U.S. Steel's production capacity, if they get the approval to merge.

The union, which has opposed U.S. Steel's merger with the Japanese steel giant, said Nippon's offer fails to commit to sustaining production over the long term or enhancing domestic capacity in integrated facilities.

"Protecting capacity only means moth-balling our equipment, allowing it to rust away to the point that it is no longer feasible to re-start," the union said in a statement on its website on Thursday.

The proposal "is nothing but a "Hail Mary" pass destined to fall to the ground," it added.

On Tuesday, reports said Nippon Steel made a proposal to give the government a final say over any potential production cuts, as part of its efforts to secure President Joe Biden's approval for acquiring the American steelmaker.

In 2023, Nippon clinched the deal to buy U.S. Steel at a hefty premium, but the merger has since faced opposition from the powerful Steelworkers' union as well as politicians.

"The truth remains that this transaction is the best way to ensure that U.S. Steel, including its employees, communities, and customers, will thrive well into the future," U.S. Steel said on Thursday in response to the union's statement.

The Committee on Foreign Investment in the United States (CFIUS) has referred the decision to approve or block the deal to Biden, who must decide on the deal by Jan. 7.

If he takes no action, it would result in the merger's automatic approval.

Nippon Steel did not immediately respond to Reuters' request for comments.

(Reporting by Shivansh Tiwary and Anshuman Tripathy in Bengaluru; Editing by Shinjini Ganguli)