He said: "They are, in essence, special troops of the Russian army - despite the fact that mercenaries and private military companies are officially banned in Russia".
Russia's Defense Ministry said its troops were not involved in the incident while Kremlin spokesman Dmitry Peskov would not comment on the issue and said the report needed to be verified.
The Pentagon said they were created to protect USA backed rebel fighters in Syria on Saturday against forces loyal to dictator Bashar al-Assad - which are in turn supported by Russian president Vladimir Putin.
Mattis, speaking to reporters Tuesday while traveling in Europe, was adamant he knew of no Russian contractors killed in the fighting, which he attributed to a surprising assault in light of obvious us advantages, including overwhelming air power. "But Russia will pretend nothing happened".
Statements at the time, carried by Syrian television stations, called the bombing a "new aggression" and "an attempt to support terrorism" with pro-government reporters describing those killed as "local people fighting (Islamic State) and the SDF".
Scores of Russian soldiers fighting as mercenaries in Syria were killed last week in an attack on a base held by United States and Kurdish forces in the country's Deir Ezzor region, Bloomberg reported on Tuesday.
He added no USA personnel were killed or wounded, though one member of the Syrian Democratic forces was injured.
"Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted, unprovoked attack", Veale said.
Most of the fatalities were attributed to an American airstrike on enemy columns that was called in by American-backed Kurdish soldiers who believed they were under attack.
US drone destroys Russian tank in eastern Syria's Deir el-Zor
Russian Foreign Minister Sergei Lavrov accused on Tuesday the United States of planning a long-term stay in Syria, charging it with trying to maintain control over the eastern part of the country.
It's unclear who had been paying the Russian mercenaries - whether it was Russia directly, Syria or a third party remains unknown. Their deaths are not listed as casualties by the Russian government, meaning the true cost of Russia's intervention could be higher than the official count.
The U.S.is in talks with Russian Federation now in search of an explanation for what happened, said two administration officials who asked not to be identified discussing private conversations.
The tank was not firing on the SDF and the coalition, although Dillon said other weapons were used.
"It's perplexing why they would do it", Mattis said. USA officials also said the Russian government had lodged no complaint about its citizens being killed.
Russian Federation signed a "road map" agreement with Assad's government last month to assist in rebuilding the nation's electricity network. Ankara has repeatedly warned against the repercussions of using one terrorist group to defeat another, while the USA has been touting the "effective results" of its cooperation with the YPG in the fight against Daesh. "We are not looking for a fight with anyone else".
At no point, a US military spokesman said, was there any chance of direct conflict between USA and Russian ground forces.
Many of the Russians killed or injured were veterans of the Ukraine conflict, according to Alexander Ionov, the head of a Kremlin-funded organization that fosters ties to separatists who's fought alongside pro-Assad forces in Syria. "I can't give you anything on that, we have not received that word at Central Command or the Pentagon", he said from Europe.
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