Argentine Vice President Amado Boudou on Tuesday urged US companies to invest in YPF, the state-owned oil company that Argentina recently expropriated from Spain's Repsol. "We are very optimistic in terms of what is coming for the Argentine economy in general and the hydrocarbons sector specifically," Boudou said at a Conference on the Americas at the US State Department in Washington. Far from scaring off foreign investment because of the expropriation, the government of President Cristina Kirchner has set the framework for "excellent opportunities for those who want to invest in joint ventures and possibilities of joint work in the energy sector," he said. The Kirchner administration is gambling that the discovery in May 2011 of a giant oilfield in Argentina's Patagonia would be too tempting for foreign oil giants to ignore. YPF needs the know-how and the capital to fully exploit the oil fields in the south-western Nequen province, known as Vaca Muerta (Dead Cow), which according to official estimates holds 150 million barrels of oil. YPF is "open to capital and the possibility of working together with public or private companies in Argentina or abroad," Boudou said. Kirchner on Friday signed a bill expropriating 51 percent of YPF's stock from Repsol, its majority shareholder, sealing a measure that has roiled the country's trade ties with Europe. Kirchner has argued that the move was justified because Argentina faces sharp rises in its bill for imported oil, and Repsol has failed to make agreed investments needed to expand domestic production. In Madrid, a Repsol spokesman Tuesday said the company has warned its competitors that they will face legal action if they invest in YPF. "The idea is to protect the assets that were confiscated in Argentina until the situation is resolved in a satisfactory way for the parties that are involved," the spokesman said. Companies that Repsol warned include Exxon Mobil, Chevron and ConocoPhillips, according to a source close to the dossier. The Repsol spokesman would not confirm the names of the companies.