"The government takeover clouds the investment climate for international companies that might otherwise have been attracted to unconventional resources in the Vaca Muerta and other plays," the agency said in its monthly oil market report.

The Vaca Muerta formation is one of the world's largest reserves of shale oil and gas recently discovered in Argentina and its development is expected to be hurt by the nationalization of the YPF by the Buenos Aires government.

The IEA, which advises 28 industrialized nations on energy policy, also said the takeover could deepen Argentina's oil products import needs in the short and medium term.

Argentina imported fuel worth $9.4 billion last year as its local crude and product production fell, according to the IEA.

"The decision to expropriate Repsol's share, rather than taking control of the share through the open market, is bound to negatively influence foreign investors' decisions in Argentina," the IEA said. "Looking forward, Argentina will need continued investment and an improved regulatory framework to turn resources into reserves."

The Vaca Muerta formation, in which YPF holds a 40 percent stake, contains around 21.2 billion barrels of oil equivalent of hydrocarbon prospective resources, according to independent estimates by energy consultants Ryder Scott.

(Reporting by Zaida Espana; editing by James Jukwey)

• A Permanent Economic Emergency