HOUSTON, May 25 (Reuters) - Saudi Aramco plans to spend $18 billion over the next five years to expand its operations in the Americas, focusing on its U.S. oil refining subsidiary Motiva Enterprises, Motiva said on Thursday.

Motiva is exploring opportunities to increase refining capacity, branch into chemicals and expand its commercial operations, marketing and branded presence over the next five years, the company said in a statement.

Motiva became a wholly owned subsidiary of Saudi Aramco on May 1 with the split of a 19-year partnership between Aramco and Royal Dutch Shell Plc.

Aramco-owned Motiva emerged from the breakup with full ownership of a refinery in Port Arthur, Texas, which is the largest by volume in the United States. It also retained the Motiva name, distribution operations across seven U.S. states and rights to use the Shell and 76 brand names on products.

"Motiva has made significant strides over the last three years to reposition our business through focused improvement efforts and organic growth opportunities," said Motiva President and Chief Executive Dan Romasko.

Thursday's announcement did not say if it was intended to supersede Saturday's similar announcement of investments that were part of the Saudi-U.S. CEO Forum. At that time, the Saudi state-oil giant said it planned an initial investment of $12 billion in Motiva with a likely $18 billion to follow by 2023.

That forum coincided with a summit between U.S. President Donald Trump and Saudi King Salman in Riyadh, Saudi Arabia. The press release remained on the Summit's website as of Thursday.

A Motiva representative did not reply to questions about the differences in investment between the two releases.

On Thursday, Motiva said "it has embarked on a growth journey to become the safest and most profitable downstream business in the U.S."

Since the completion of the expansion of the Port Arthur refinery in 2012, which more than doubled its capacity to refine 603,000 barrels per day of crude oil, Motiva has weighed plans for further expansion of the plant.

Saudi Aramco has also looked at acquiring at least one additional Gulf Coast refinery and visited chemical plants up for sale to expand Motiva's portfolio.

U.S. refiners preparing for domestic gasoline demand to peak within 20 to 30 years are looking at increasing exports of diesel and jet fuel and expanding petrochemical production.

(Reporting by Erwin Seba; Editing by Richard Chang and James Dalgleish) ((erwin.seba@thomsonreuters.com; +1 713 210 8508; Reuters Messaging: erwin.seba.thomsonreuters.com@reuters.net))