Argentina's Bridas Group and China National Offshore Oil Corporation (CNOOC) are suing Spanish giant Repsol to prevent it from blocking a $1.5 billion shale-gas project, according to reports.

The group's Virgin Islands-based Bridas International and Bridas Energy Holdings units filed the lawsuit against Repsol on 3 January in New York State Supreme Court in Manhattan, saying the Spanish oil producer sent them letters threatening to seek to block the deal with YPF, Bloomberg reported, citing court documents.

The Argentinean government seized control of Repsol's majority stake in YPF in April last year, accusing Repsol of investing too little to boost output at a time of growing demand.

"Bridas's suit has no merit, it is trying to delay the inevitable when it comes to our legal action," Bloomberg quoted Repsol spokesperson Kristian Rix as saying.

This lawsuit has made Repsol "take notice" and they expect to take legal action in the coming days, Bloomberg reported.

Bridas wants a court order stating that the question of YPF's ownership is for Argentina's court to decide, according to the lawsuit.

YPF and Bridas agreed on 28 December to invest $1.5 billion to develop shale oil reserves in Patagonia. The partnership plans to drill 130 wells. Bridas has promised to provide as much as $500 million in financing for YPF, "to be repaid in 10 years," according to the court documents filed in New York.

YPF has granted Bridas access to Argentina's official currency market for the "payments of imports, debt and interest repayment and dividends to Bridas's satisfaction," according to the documents. To stem capital outflows, Fernandez last year banned most purchases of foreign currency by individuals.

Bloomberg noted that a spokesperson for YPF did not respond to calls or e-mails from the news agency for comment.

Earlier this month Repsol vowed to take legal action against Bridas for unlawful competition under Spanish law for partnering with YPF. 

Repsol has already launched similar legal action against Chevron in a federal court in Manhattan, claiming the US supermajor improperly obtained rights to develop Argentine shale and natural gas resources.