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

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.

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 the company's midstream business.