The activist investor plans to push the oil refiner to consider selling or spinning off its energy-transportation business.

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

(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 the company's midstream business.