Adds Elliott response in paragraph 8, reflects letter was published in paragraphs 1-2

By David French

April 21 (Reuters) - Activist investor Elliott Investment Management should back down from its push to break up energy company Phillips 66 PSX.N because it is conflicted from a separate effort to acquire one of the group's rivals, Phillips 66 said in a letter on Monday.

The salvo is the latest in a bitter spat between Phillips 66 and Elliott that is due to come to a head at a shareholder meeting next month.

In arguing against Elliott's break-up thesis in the letter, Phillips 66 said the investment firm has a conflict of interest due to its separate efforts to buy Citgo Petroleum.

Citgo's parent company is being sold via a court-supervised auction. Last year, Elliott-backed Amber Energy was initially deemed the winner of the process, before creditor challenges forced the court to backtrack and launch a new sale.

Amber Energy CEO Gregory Goff said on April 9 he had bought a position in Phillips 66 and backed Elliott's campaign.

"This conflict is concerning because Amber Energy's executives are actively helping support Elliott's case to undermine Phillips 66's strategy," said Monday's letter from Phillips 66.

Phillips 66 is one of the largest U.S. refiners, while Citgo is the seventh largest.

In response to a request for comment, an Elliott spokesperson pointed to an April 10 regulatory filing.

This stated that Goff's work alongside Elliott was "hidden from no one, and in no way represents a conflict of interest, diminishes the independence of his views or impairs his ability to help Phillips 66 become a stronger, more valuable company".

Elliott has put forward four director nominees for the May 21 meeting as part of a campaign that has also included calling on Phillips 66 to sell or spin off its midstream business and consider divesting other assets to focus on its refining business and boost its share price.

Elliott has said it has a more than $2.5 billion investment in Phillips 66. It is the second time the investment firm has pushed for change at Phillips 66, after a first effort ended in early 2024 with the addition of a new company board member blessed by Elliott.

(Reporting by David French in New York; Editing by Leslie Adler and Jan Harvey)

((davidj.french@thomsonreuters.com;))