By Arathy Somasekhar

HOUSTON (Reuters) - Activist investor Elliott Investment Management has accepted the performance improvement plan that U.S. oil refiner Phillips 66 laid out to boost shareholder returns and share price, Chief Executive Mark Lashier said in an interview on Tuesday.

"They've bought into our plans that we already had in place," the CEO of one of the largest U.S. oil refiners said in a briefing at its Houston headquarters.

Elliott sent a letter to the company's board last fall, disclosing a $1 billion stake in the company, and calling for additions to its board of directors and a focus on improving its oil refining business.

Since the activist firm publicly disclosed its recommendations, Phillips 66 shares have climbed 32%, to $156.37 per share, compared to a 15% increase in the S&P 500 Index.

"Elliott sees the progress. I think they've done quite well as any investor has that entered the shares over the last couple of years," he said.

Last month, Phillips 66 appointed Robert Pease, a veteran refining executive as a director, and said it was looking to add a second candidate to the 14-person board. Elliott had urged the company last fall to add directors with refining experience that could address underperformance in refining and speed up cost-cutting efforts.

The oil refiner and pipeline operator's performance improved in 2023's fourth quarter with higher margin capture in refining and gains by its pipeline unit after a two-year period in which the company's overall earnings lagged rivals.

(Reporting by Arathy Somasekhar)