The decision by Fernandez "is only a way of covering up the social and economic crisis Argentina is facing" amid high inflation and energy prices, Brufau added.
Repsol shares were down 7.1 percent to €16.25 ($21.34) each in afternoon trading in Madrid, far underperforming the benchmark Ibex index, which was up 1 percent. Analysts were concerned that Argentina has not stated any compensation terms for the nationalization of YPF, which has 42 percent of Repsol's global reserves, estimated at 2.1 billion barrels of crude.
Brufau told reporters that YPF is worth $18.3 billion, and he valued Repsol's 57 percent stake in the unit at $10.5 billion. With Argentina taking over Repsol shares representing 51 percent of YPF, Repsol would be left with 6 percent. Other shareholders including a rich Argentine family would not be affected, and other foreign energy companies in Argentina operations were not targeted, he noted.
Kicillof, however, gave other energy companies few reassurances on Tuesday. In his Senate speech, he asserted that all of them need to be subject to state control.
Spain's Justice Minister Alberto Ruiz Gallardon said Argentina's plan to nationalize YPF "represents an extraordinary political error in the medium and long term." He and Energy Minister Jose Manuel Soria suggested that retaliatory measures would be taken within days.
Bilateral accords between Spain and Argentina allow Repsol to take its case to the United Nations and the World Bank. "It will be a long (legal) battle," Brufau warned.
YPF is Argentina's largest company and vital for its energy future, especially after a recent find of huge unconventional oil and natural gas reserves — a discovery that Brufau stressed came from his company's exploration efforts.
But the company has been under intense pressure from Fernandez' government to raise output while its shares have plunged in recent months on fears of possible state intervention.
Argentina this year expects to import more than $10 billion worth of gas and natural liquid gas to address an energy crisis even though it is an oil-producing nation, according to estimates from the hydrocarbon sector.
Spain's government is lining up allies to contest the nationalization and possibly isolate Argentina economically. Rajoy was already finding support during his trip to Mexico. Venezuela, meanwhile, pledged to use all means necessary to support Argentina.
Brufau urged shareholders to join the legal battle, and accused Fernandez of being "an expert manipulator" in her accusations that Repsol underfunded its YPF unit. He said Repsol has invested $20 billion in Argentina since it bought its stake in YPF in 1999.
As Fernandez was announcing the takeover on national television, Argentine authorities went to YPF headquarters in Buenos Aires and expelled Spanish executives. Their "behavior with our managers and employees was pathetic and embarrassing," Brufau said.
Spanish bank and telecommunications companies have a heavy presence in Argentina, where they also have earned strong profits to offset deep losses in recent years at home due to the financial crisis.
Spain's Telefonica SA operates six companies in Argentina, where it is the leading telecom provider, with revenue last year of €3.17 billion, up from €3 billion in 2010, according to its annual report.
The YPF nationalization has made the business climate in Argentina more uncertain for foreign companies, but experts doubted Fernandez would announce nationalizations affecting others.
"There's going to be a lot of mounting pressure on Argentina not to do more of this," said Antonio Moreno, an economics professor at the University of Navarra. "I don't think they're going to be able to go on a nationalization spree."
Contributing to this report were Associated Press writers Luis Andres Henao in Buenos Aires; Alan Clendenning, Jorge Sainz and Ciaran Giles in Madrid; Jack Chang in Puerto Vallarta, Mexico; Barry Hatton in Lisbon, Portugal, and Gabriele Steinhauser in Brussels.
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