Argentina's decision to nationalize a part of Spanish oil giant Repsol sends a "very negative signal" to global investors, EU foreign affairs chief Catherine Ashton said Tuesday, April 17.

Citing "cause for grave concern," the European Union's top international representative said at the European Parliament that the decision would alarm international financiers "and could seriously harm investor confidence (in) Argentina."

Repsol has vowed to fight for at least $10 billion (8.0 billion euros) in compensation for the expropriation of subsidiary YPF, which was announced overnight in Buenos Aires by President Cristina Kirchner.

"These acts will not remain unpunished," Repsol executive chairman Antonio Brufau said earlier as shares in his company plummeted more than seven percent on the Madrid stock market.

After European Commission president Jose Manuel Barroso said he was "seriously disappointed" by the decision, Ashton said the action creates "legal insecurity for all European Union and foreign firms in the country."

She added she was all the more alarmed by threats to further international investments in telecoms and banking, and said "all possible options are being analyzed," according to EUbusiness.