U.S. mercenaries are not involved in suppressing riots in southeastern Ukraine, the White House said on Tuesday, citing the Greystone private military organization.

"I would point you to what the company [Greystone] itself has said in the past, which is that this is not true," White House press secretary Jay Carney said at a press conference.

As reported, the Russian Foreign Ministry said on April 7 that units of Ukraine's Internal Troops and National Guard, as well as fighters of the ultra-nationalistic Right Sector and U.S. specialists from a privately owned military organization, were sent to the southeastern regions of Ukraine.

"We are particularly troubled by the circumstance that some 150 American specialists from the privately owned military organization Greystone dressed in the Sokil [police] unit's uniforms have joined this operation," the ministry said.

Meanwhile, U.S. newspaper Wall Street Journal reported, citing Greystone representative Coreena Taylor that the company's specialists were not working in Ukraine.

"We do not have anyone working in Ukraine nor do we have any plans to deploy anyone to the region," she said.