President-elect Donald Trump's meetings with CEOs seeking federal approval for major mergers are raising red flags for ethics lawyers concerned that about the possible erosion of a firewall between the regulators tasked with approving the billion-dollar deals and the White House.

Trump met this week with the heads of German chemical company Bayer and seed and herbicide giant Monsanto, who made their case for their $US57 billion merger. The deal would likely need to be approved by Trump's choices to lead antitrust enforcement at the Justice Department. On Thursday, Trump sat down to discuss jobs with the chief executive of AT&T, which is trying to acquire Time Warner.

Presidents typically keep their distance from such reviews, so as not to appear to be exerting political influence on a regulatory process intended to evaluate the impact a merger could have on competition and consumers. Trump's closed-door sessions suggest the incoming president may be less worried with appearing to be close to pending deals that require government approval.

"While it's true the Department of Justice is under the executive branch, it's not appropriate for the president to make that regulatory decision - and certainly not for political considerations," said Bruce Green, a law school professor at Fordham University who specialises in ethics.

Green equated the meetings to a 2016 campaign controversy: Bill Clinton's conversation with Attorney-General Loretta Lynch on the Phoenix airport tarmac at a time when the Justice Department was looking into Hillary Clinton's use of a private email server.

"If the conversation is private, it will raise questions and suspicions," Green said.

Part of the challenge is not knowing what was precisely said at the meetings.

"We don't know really what they were discussing, what Trump's response was to that and to what extent that will influence the antitrust review," said Maurice Stucke, a former attorney in the Justice Department's antitrust division who teaches at the University of Tennessee College of Law.

On Wednesday, Werner Baumann, Bayer CEO, and Hugh Grant, Monsanto CEO, talked about their merger as in the broader conversation on innovation, Monsanto said in a statement.

AT&T specifically denied talking about its proposed $US85.4 billion merger when CEO Randall Stephenson met on Thursday with Trump. The company said its conversation focused on how it could increase its US investments, create jobs and make American companies more competitive.

Trump's choice for attorney-general told senators this week that incoming administration would be transparent.

"The antitrust policies of the United States have to be consistent and as clear as possible," Alabama Senator Jeff Sessions said at his confirmation hearings.

"I have no hesitation, if the finding justifies it, to say that certain mergers should not occur and there will not be political influence in that process," he said.

During the campaign, Trump opposed the combination of the telecom AT&T and Time Warner, the media conglomerate that owns HBO and CNN. "It's too much concentration of power in the hands of too few," Trump said at the time.

The meetings are part of Trump's aggressive and unorthodox strategy for job creation, in which he openly cheers on, and sometimes jeers, individual companies.