Activist investor Elliott Investment Management LP has built up a more than $2.5 billion stake in Phillips 66, and is looking for the oil company to either sell or spin off its midstream unit to help boost its stock price.

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 and the sale of the company's midstream business.

Activist Elliott Investment Management on Tuesday disclosed a stake in Phillips 66 valued at more than $2.5 billion and said it is pushing the oil refiner to consider operational changes to boost its stagnant stock.