(Adds comment from Mattis, U.S. official)

MOSCOW, Feb 13 (Reuters) - The Kremlin said on Tuesday it had no information about Russian mercenaries reportedly being killed in Syria, saying it only knew about Russian nationals who had been deployed there as members of the Russian armed forces.

It was responding to media reports citing associates of Russian mercenaries saying some of them had been killed this month when U.S.-led coalition forces clashed with pro-government forces in Syria's Deir al-Zor province.

"We don't have information about other Russians who might be in Syria," Kremlin spokesman Dmitry Peskov told reporters on a conference call when asked about the alleged deaths.

Speaking with reporters in Europe, U.S. Defense Secretary Jim Mattis said he had seen the media reports.

"There is now reporting in the press. I don't have any reporting that some Russians, non-Russian Federation soldiers, but Russian contractors, were among the casualties. I can't give you anything on that, we have not received that word at Central Command or the Pentagon," Mattis said.

In Washington, Lieutenant General Jeffrey Harrigian, the top U.S. Air Force general in the Middle East, said he would not speculate on the forces.

"I will not speculate on the composition of this force or whose control they were under," Harrigian told reporters. (Reporting by Polina Nikolskaya in Moscow, Idrees Ali in Brussels and Phil Stewart in Washington; Editing by Andrew Osborn/Mark Heinrich)