Russian media said the contractors were part of pro-Assad forces that advanced on oil fields in the eastern Deir el-Zour province and were targeted by the United States.
The deadly fight began about 8 kilometers (5 miles) east of the Euphrates River de-confliction line late on February 7, when adversaries fired rounds and advanced in a "battalion-sized dismounted formation supported by artillery, tanks, multiple-launch rocket systems and mortars", Veale said.
The Russian Defense Ministry said its troops weren't involved in the operation.
The reports follow one from Reuters on Monday, where former associates of two of the Russians killed identified them to the news wire.
Already Grigory Yavlinsky, a veteran liberal politician who is running for president in elections next month, has called on Putin to disclose how many Russians had been killed in Syria and in what circumstances.
Leviev said his group has confirmed the death of eight Russian private military contractors in the US strike based on social media.
"The Russians told us they had no forces there initially". The Trump administration is under investigation for alleged collusion with Moscow during the USA presidential campaign to tilt the election in Trump's favor, and a Senate committee found that Russian Federation had acted to influence the 2016 U.S. presidential election.
Asked at a Senate Intelligence Committee hearing about Russian deaths, CIA Director Mike Pompeo said, "From an intelligence perspective, we have seen in multiple instances foreign forces using mercenaries in battles that will begin to approach the United States".
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One of the Russians killed was Kirill Ananyev, according to his friend Aleksandr Averin.
Harrigian and other USA officials have declined to say which forces were responsible for last week's larger attack.
Kremlin spokesman Dmitry Peskov has told media that Moscow has "no information about the deaths of any Russian mercenaries in Syria", adding that they only keep track of "Russian forces servicemen", and not other Russians who "could be in Syria".
President Trump (L) with Russia's President Vladimir Putin.
The Russian citizens are fighting in Syria for a private military company, according to numerous reports. "I advise you to contact the defence ministry".
In an interview with Russian Izvestia newspaper on Tuesday, Bogdanov said that the basis on which the efforts for maintaining Syria's unity and preventing all attempts to undermine it is the UN Security Council resolution no. 2254 and the basic principles of global law based on the UN charter.
Without mentioning the USA strike, Russian Foreign Minister Sergey Lavrov said Tuesday "Americans have taken risky unilateral steps". "We don't have data about other Russians who could be in Syria".
Mr Yavlinsky said fierce mercenary forces "are trained at Russian army facilities" and "receive military awards in the Kremlin" in secret. Prigozhin's assets include Evro Polis, an oil trading firm that reportedly has served as a front for Wagner's operations in Syria. 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. In recent days, rumors swirled online in Russian Federation that large numbers of those military contractors had perished under American fire.