The US Federal Bureau of Investigation is probing an attack by suspected Russian computer hackers on Citigroup Inc. that resulted in the theft of tens of millions of dollars, The Wall Street Journal reported Tuesday.

The newspaper, citing unidentified government officials, said the attack by hackers who appear to be linked to a Russian cyber gang targeted Citigroup's Citibank subsidiary.

It said the hackers also assaulted two other entities, at least one of them a US government agency.

The Citibank attack was detected over the summer, the Journal said, but investigators are looking into the possibility it may have occurred months or even a year earlier.

Citigroup flatly denied any such attack had taken place.

"We had no breach of the system and there were no losses, no customer losses, no bank losses," Joe Petro, managing director of Citigroup's Security and Investigative services, told the newspaper.

"Any allegation that the FBI is working a case at Citigroup involving tens of millions of losses is just not true," he said.

The Journal said US banks have generally been loath to disclose computer attacks for fear of scaring off customers.

The newspaper said the Citibank attack appeared to have been carried out by alumni of the "Russian Business Network," a Russian gang that has sold hacking tools and software for accessing US government systems.

The Journal said the hackers had used infected computers commanded by software called Black Energy, which was also used during Russia's brief 2008 war with Georgia to shut down Georgian government and bank websites.

Black Energy is usually used to block websites but can be upgraded to invade computer systems and snatch data, the Journal said.

The US Justice Department last month indicted alleged computer hackers from Estonia, Russia and Moldova in connection with a scheme that netted nine million dollars from cash dispensers.

They were accused of hacking into a computer network in November 2008 operated by Atlanta-based credit card processing company RBS WorldPay, part of the Royal Bank of Scotland.