(Updates sourcing, adds context)

LONDON/MADRID, May 24 (Reuters) - State-controlled oil company Saudi Aramco is interested in buying a minority stake in the renewable unit of Spanish oil company Repsol, two people familiar with the matter told Reuters.

The Spanish company has opened its renewable projects to investors willing to take minority stakes in portfolios of wind farms and solar plants to help fund its diversification into renewables and low carbon businesses, away from its traditional oil and gas business.

Repsol Renewables is valued at 5.9 billion euros ($6.40 billion), including debt, according to a research note published by UBS in April.

Spanish newspaper Expansion reported earlier on Friday that Aramco had approached Repsol.

Saudi Aramco has approached Repsol as it is mostly interested in the Spanish company's renewable assets in the U.S. The Saudi giant had not filed a formal offer yet, Expansion said.

Repsol has started talks to sell a stake in its renewable energy business after receiving an unsolicited approach by an investor and has lined up Santander as advisor for the sale, four sources told Reuters earlier this month.

Two years ago, Repsol sold a 25% in the business to a unit of French bank Credit Agricole and Swiss asset manager Energy Infrastructure Partners for 905 million euros ($978.31 million).

Saudi Aramco did not reply to a request for comment out of regular office hours. Repsol declined to comment. ($1 = 0.9219 euros) (Reporting by Inti Landauro; editing by David Evans and Sharon Singleton)