TOKYO, Nov 7 (Reuters) - Japan's Nippon Steel (5401.T), opens new tab hopes to close its $15 billion deal to buy US Steel (X.N) by year-end before Republican Donald Trump retakes the White House in January, a senior executive said, as the acquisition is core to its future growth.

Nippon Steel, Japan's top steelmaker and the fourth largest globally, agreed to buy US Steel in December but faced opposition from a powerful labour union as well as from Trump, and President Joe Biden, among others.

Before the Nov. 5 election which handed him victory, Trump promised to block the deal if re-elected.

"We believe we can close the US Steel deal by the end of the year under the current U.S. administration," Takahiro Mori, Nippon Steel's vice chairman and key negotiator on the deal, told reporters on Thursday.

The Committee on Foreign Investment in the United States (CFIUS) has extended its review of the $15 billion deal until the end of December. Mori said on Thursday that CFIUS' and antitrust authorities' reviews were proceeding 'solemnly'.

"Now that the U.S. election is over and we are in a position to have a proper discussion, there is no reason to postpone the review process any longer," Mori added.

In order to win the approval, Nippon Steel has made social guarantee and investment pledges to US Steel and the United Steelworkers labour union, and promised to sell a stake in a U.S. steel plant's joint venture if it succeeds in the buyout.

"The United States is essential to our continued growth as it is the world's largest market for high-grade steel and is a growing market with little China risk," Mori said.

With US Steel, Nippon Steel aims to raise its global steel production capacity to 85 million metric tons per year from 65 million tons now, and the asset is core to its goal of having over 100 million tons of production capacity in the long-term.

Mori, having paid numerous visits to the US seeking approval for the deal, believes that the takeover is in line with Trump's goal of attracting investments, adding that his company needs to build close ties with the new administration.

Earlier this year, Nippon Steel hired former U.S. Secretary of State Mike Pompeo, who served under Trump's first presidency in 2017-2021, as an adviser to help in lobbying for the deal.

On Thursday, Nippon Steel revised its full-year net profit outlook to 310 billion yen ($2 billion) from a previous forecast of 340 billion yen due to inventory losses from weak raw material prices and sluggish domestic demand.

Even as Nippon Steel revised its forecast for the fiscal year ending March, it expects to maintain its full-year dividend target at 160 yen apiece. Its net profit for the six-month period ended Sept. 30 was down 19% at 243.4 billion yen year-on-year.