Spanish oil company Repsol threatened on Monday legal action against any investors helping newly nationalized Argentine oil company YPF develop oil resources previously discovered by the Spanish firm.

Repsol and the Argentinian government have been at odds ever since the South American country nationalized YPF, in which Repsol owned a majority share.

Argentina is in the midst of an energy development boom, with vast tracts of natural gas found in Patagonia and in the center of the country.

Recent findings suggest Argentina holds the world's third largest shale natural gas reserves in the world, Reuters reported.

Argentine government officials last week seized YPF after accusing Repsol of not doing enough to develop the South American country's oil and natural gas resources.

The seizure has complicated relations between Madrid and Buenos Aires; the former announced any move against Repsol would be considered an act of aggression.

A spokesman for Repsol said on Monday the company could seek legal action against those planning to develop energy deposits previously targeted by Repsol.

The company is reserving the rights to take legal action against any investor in YPF or its assets following the unlawful expropriation of that company, CBS reported.

Argentina is trying to get other major oil companies to invest in YPF. Argentina's Planning Minister Julio de Vido asked Petrobras, Brazil's state-run oil company, to invest; Bloomberg reported the government could also approach Exxon Mobil Corp. (NYSE: XOM), Chevron Corp. (CVX) and ConocoPhillips (COPS).