Feb 11 (Reuters) - Elliott Investment Management said on

Tuesday it has built a stake of more than $2.5 billion in U.S.

refiner Phillips 66, and plans to push for operational

changes as well as the sale of its midstream business.

In March, the activist investor disclosed a $1 billion stake

in the company, following which Phillips 66 laid out a

performance improvement plan to boost shareholder returns and

share price.

Elliott said that shares of Phillips 66 have lagged that of

rival refiners, and they are trading at a substantial discount.

Phillips 66 shares rose 6.6% in premarket trading on

Tuesday.