However, according to Russian officials, there was initial information about the death of five citizens but they had no relation to Russia's armed forces.
Her oldest son and five other men left to fight in Syria as military contractors last October, she said, with at least one of them killed and the whereabouts of her son unknown.
The war in Syria and the invasion that Russian Federation made in some parts of Ukraine, have made the already traditional struggle between Washington and Moscow even more tense. When Mattis and other USA officials publicly discuss the US mission in Syria, they say only that it is to fight the Islamic State.
In December, Russian President Vladimir Putin traveled to a Russian air base in Syria and disclosed plans to send troops home. The Pentagon says they don't think the Russian government knew what was going on.
Hockey sur glace (H) : le Canada tombe face aux Tchèques
Les deux équipes n'ont pas été en mesure de se départager en prolongation et sont donc allées aux tirs au but. Le Canada s'était incliné 5-3 face aux États-Unis dans son dernier match de groupe.
The Russian defense ministry did not respond to questions from Reuters about casualties in Syria.
Reuters was unable to make direct contact with the contractors' employers, the Wagner Group, whose fallen fighters have in the past received medals from the Kremlin. Predominant anti-Russian tendencies in Washington's policies will scarcely subside before the midterms scheduled for November, Russian Foreign Minister Sergei Lavrov said in an interview with Euronews channel on Friday. "I don't have any reporting that some Russians, non-Russian Federation soldiers, but Russian contractors, were among the casualties". It said that any Russians fighting alongside the Syrian forces were mercenaries and that no member of the Russian military was killed. He said the United States "doesn't want to be killing Russian citizens in Syria".
He said the wounded men had told him that the two units of Russian contractors involved in the battle near Deir al-Zor numbered 550 men.
Asked on February 14 whether Putin had canceled trips and meetings that had been scheduled in the past few days in order to discuss the reports of Russian deaths in Syria, Peskov said that was not the reason.
Moscow helped turn the tide of the conflict in his favor by launching a campaign of air strikes in 2015 and stepping up its military presence on the ground.
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A Reuters reporter visited the Burdenko hospital on Wednesday and spoke briefly to patients who said they knew nothing about anyone evacuated from Syria.
"Everybody understands the need for a law", said Mikhail Yemelyanov, an MP in the Just Russia party, who is one of the authors of a bill on private military companies now being reviewed by government.
"The U.S. continues military occupation of 55-kilometer zone around Al Tanf, that is now, in fact, a security zone for the remnants of Daesh", she said.
"I will not speculate on the composition of this force or whose control they were under", said Lieutenant General Jeffrey Harrigian, a spokesman for U.S. Central Command.
See Vibrant Photos of Chinese New Year Celebrations All Over the World
Victoria's Chinatown will come alive with the sights and sounds of Chinese New Year as they welcome the Year of the Dog. Bye, Roosters! The previous Lunar Year belonged to the feathered friend of the farm but their time is over.