TOKYO, March 19 (Reuters) - Nippon Steel would move its U.S. headquarters to Pittsburgh where its acquisition target U.S. Steel is based if the deal goes through, an executive said, adding to the pledges from the Japanese steelmaker to secure the transaction.

Nippon Steel's proposed $15-billion takeover of U.S. Steel has drawn sharp criticism in the United States where President Joe Biden said last week the asset should remain domestically owned while his opponent in November presidential vote, Donald Trump, promised to block the deal if he is re-elected.

The Japanese company, the world's fourth biggest steelmaker, has been in talks with the United Steelworkers (USW), a labour union key for Biden and Trump in the upcoming November elections and which so far was opposing the deal.

"Not only are we keeping the Pittsburgh headquarters - which other bidders would not be able to do - but we are planning to move Nippon Steel's existing U.S. headquarters from Houston to Pittsburgh," Executive Vice President Tahahiro Mori, Nippon Steel's key negotiator on the deal, was quoted as saying.

In comments released in a U.S. Steel document to the U.S. Securities and Exchange Commission overnight, Mori said his company remained engaged with the USW and has offered a number of specific commitments on job security, investments and others.

Nippon Steel would honor all agreements currently in place between the U.S. Steel and the USW, will not cut jobs, close facilities or move more production overseas, Mori reiterated. (Reporting by Katya Golubkova; Editing by Lincoln Feast.)