Mexican state-owned oil company Petroleos Mexicanos slammed two multinationals it is suing in U.S. federal court for bribery, saying they have spread "false information" and conducted a media campaign designed to sway public opinion.

In December, Pemex filed a $500 million suit before the U.S. District Court for the Southern District of New York, accusing Siemens AG and South Korea-based SK Engineering & Construction Co. of bribing company executives to overlook cost overruns in a project to overhaul the Cadereyta refinery.

Siemens and SK, which made up the Conproca consortium responsible for the refinery upgrade, "have spread false information about the judicial process stemming from the suit filed by Pemex," the Mexican oil giant said in a statement Tuesday.

Pemex said the companies' media campaign "seeks to have the suit filed against them dismissed and mislead public opinion."

The company recalled that the defendants recently petitioned the judge in the case to throw out the lawsuit.

Pemex said Siemens and SK Engineering "developed and carried out a system of submitting fraudulent bills that were sent by Conproca to a trust in New York for payment."

This system, it said, was aimed at covering the financial shortfall Conproca incurred after presenting Pemex a below-market bid for the refinery. The Mexican company said Conproca, after winning the contract, manufactured a series of cost overruns and bribed some Pemex officials to secure payment of invoices.

In response, Conproca said in a statement Tuesday that Pemex had changed its version of the events on several occasions and "in no instance has presented irrefutable evidence to support its allegations."

The consortium "categorically" denied that any invoices it submitted were fraudulent.

Pemex, the world's fifth-largest oil company by output, has a monopoly on the production of oil and natural gas and refined products in Mexico. EFE