Nippon Steel has made a pledge to workers at US Steel to protect their jobs and benefits if it acquires the company. The Japanese firm expressed this commitment in a letter addressed to the workers.

The letter pledges to make the US steelmaker more competitive and keep its blast furnace operating over the long term.

Nippon Steel says it will refrain from interfering in the US firm's trade measures in tackling low-priced imports.

The letter also says the planned acquisition would help strengthen US national security.

The two steelmakers reached an agreement on the buyout in December last year. But the union representing the workers has been opposing the sale.

The United Steelworkers union issued a number of letters to its members this month. It claims the acquisition would threaten US national security and the investment in US Steel would not be legally binding.

Nippon Steel's letter claims the union is not seriously considering its proposals. The letter urges union leaders to agree to negotiations.