TOKYO, Sept 4 (Reuters) - Japan's biggest steelmaker Nippon Steel (5401.T), opens new tab, which is seeking to acquire U.S. Steel (X.N), opens new tab, said on Wednesday if its purchase goes ahead core senior management as well as a majority of board members at the U.S. company would be U.S. citizens.

The announcement follows Democratic presidential candidate Kamala Harris this week echoing President Joe Biden's concern over the $15 billion deal, saying that U.S. Steel "should remain American-owned and American-operated".

Nippon Steel said on Wednesday that U.S. citizens would make up the majority of the board of directors of U.S. Steel which will also include three independent directors who would be U.S. citizens as well. Core senior management members would be U.S. citizens too, the Japanese firm said.

U.S. Steel would be owned by Nippon Steel North America, a New York-based unit of the Japanese steelmaker which has been operating in the U.S. for over 50 years, Nippon Steel said.

To gain support from politicians and labour unions, Nippon Steel hired former U.S. Secretary of State Mike Pompeo as an advisor and unveiled a plan last week to invest an additional $1.3 billion in U.S. Steel's mills.

Nippon Steel said its Vice Chairman Takahiro Mori, a key negotiator on the deal, plans to visit the United States this month to continue deal-related meetings but declined to specify dates or disclose the names of those Mori is set to meet.

Both companies aim to close the deal by the end of the year depending on regulatory approvals.