It marks the first time Russian and USA combatants have become directly engaged in combat in Syria, according to the outlet.
"Based on preliminary information, as a result of an armed encounter, the reasons for which are now being investigated, we could be talking about the deaths of five people", the ministry's spokesperson Maria Zakharova said.
At least six Russian mercenaries - and reportedly scores more - who were fighting on the side of Bashar al-Assad, died in the U.S. strikes, multiple reports from Russia suggest.
Russian Federation security staff were killed at a weapons depot in the area. It said it was in regular communication with Russian counterparts before, during and after the attack, and Russian officials assured coalition officials they would not strike coalition forces.
Officials in both Moscow and Washington remained coy about the deaths, which would be an embarrassment for both countries if it turns out that Russian fighters were part of a unit that attacked a military force with American troops. It said USA advisers had a presence among the opposition group.
"We do think that this is a direct and obvious attempt to time some sort of action to coincide with our elections in order to influence them", Kremlin spokesman Dmitry Peskov told journalists on a telephone conference call last month.
Moscow says five Russian citizens may have been killed in recent attacks by U.S. warplanes on pro-government forces in the eastern Syrian province of Dayr al-Zawr.
Earlier this week, media reports claimed that a recent United States airstrike in Syria left Russian servicemen dead, putting the figures anywhere from dozens to several hundred, RT reported.
In the days after the strike, US Defense Secretary Jim Mattis and other US officials, however, said they had no information on Russian casualties.
"Despite the attack being unprovoked, [the assault] was not entirely unexpected", he told reporters at the Pentagon this week, describing the Syrian assault near SDF positions in Deir-e-Zour.
Russian forces are supporting the Syrian government in its fight against opposition groups, some of which are backed by the United States, and elements of both sides are fighting the last remnants of the Islamic State group in Syria. There were USA troops present at the base of the Syrian Democratic Forces (SDF) rebels. Those claims could not be verified.
The Kremlin and the Russian Defense Ministry refused to comment on media reports on Russian private contractors in Syria. Officials from the Moscow-based Conflict Intelligence Team, an independent watchdog group monitoring Russian operations in Ukraine and Syria, have identified 12 Russian contractors among those killed in the strike, Radio Free Europe reported Wednesday.
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