Japan's Nippon Steel said Friday its proposed $14 billion takeover of US Steel Corp is going through an official US merger review, after a report that the Justice Department has launched an antitrust investigation.

Nippon Steel's acquisition plan has become a political issue ahead of the US presidential election, with President Joe Biden saying he was against it.

Biden and Japanese Prime Minister Fumio Kishida were asked about the deal during a news conference on Wednesday after their talks at the White House.

"The acquisition of US Steel is subject to certain regulatory approvals such as those regarding merger control and foreign investments in relevant countries," Nippon Steel said in a brief statement. 

"While we are progressing through the merger control review process in the US as well, we will not comment on specific details about the status of the review process," it said.

Nippon Steel was responding to a request for information from AFP about a report by news outlet Politico on Wednesday that said the US Justice Department had opened an "in-depth antitrust investigation".

"The move escalates US government scrutiny of the controversial deal, which has drawn fire from lawmakers, labor unions and others who argue the storied US industrial titan should not be owned by a foreign company -- even one based in Japan, one of America's closest allies," Politico said.

Asked about the deal after his meeting with Biden, Kishida said he hoped for "good discussions" between the two nations.

Biden said: "I stand by my commitment to American workers", as well as the US-Japan alliance.

US Steel is based in Pittsburgh, in the battleground state of Pennsylvania, which Biden won in the 2020 election.