(ANTIMEDIA) — Pro-government forces in Syria attacked by the U.S.-led coalition on Wednesday included “Russian mercenaries,” according to a Pentagon official, CBS reports.

If true, CBS notes, this incident would mark the first time a U.S. airstrike has killed Russians in Syria.

Update: The Russian Defense Ministry, in a statement, said there were no Russian military personnel in that particular area.

The attack on pro-government forces allegedly took place in response to an “unprovoked attack” by Syrian forces against what CNN termed “a well-established Syrian Democratic Forces headquarters where coalition advisers were working with US-backed Syrian fighters.”

CBS reports that according to Pentagon officials, Russia was not looking to attack the U.S. military presence but had its sights set on a lucrative oil field nearby, which the SDF had seized from ISIS.

The coalition claimed to have killed at least 100 Syrian troops in response to an attack that killed zero U.S. personnel or Syrian Democratic Forces (SDF) fighters.

On Thursday, U.S. Defense Secretary Jim Mattis, living up to his nickname “Mad Dog,” said the situation was “perplexing” and that he had “no idea why they would attack.” If another country had invaded the United States and set up bases of its own, one can imagine how the U.S. government would direct its military to respond. Most likely, such a scenario would not be so perplexing if the tables were turned against the U.S. government.

Mattis also said he was not aware of any Russian casualties.

Unsurprisingly, Russia immediately condemned the U.S. strike and its overall military presence in general, noting that it is illegal. Russia’s U.N. ambassador, Vassily Nebenzia, said he told the U.N. Security Council that the attacks on Syrian forces were “inadmissible” and “deplorable” and that they cannot be repeated regardless of any justification by his American counterparts.

He added that “to confront those who really fight the international terrorists on the ground, on the Syrian side, is criminal.”

In April last year, a report published by the London-based IHS Jane’s Terrorism and Insurgency Center, a leading security analysis agency, found that the Syrian government and its associated allies were the most heavily engaged force fighting ISIS on the ground in Syria. Despite the various allegations against the Assad government, what Nebenzia is saying about the U.S. targeting government troops who are in the business of fighting terrorists has some basis in reality.

It is not clear if Russia has a retaliation of its own in mind, but we would do well to remember that when similar incidents took place last year, Russia indicated that they would begin targeting U.S. jets if they had to. Almost immediately after this stark warning, Australia pulled its planes out of its air missions in Syria, a signal that the situation had, indeed, become quite volatile.

In a bid to avoid further bloodshed, the U.N. has now called for an immediate 30-day cease-fire to deliver aid and evacuate the critically ill, but the U.N. Security Council took no immediate action.

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In what can only be described as satire in real-life, the U.S. has also echoed the call for a month-long violence-free period in Syria even though they have spent billions of dollars contributing to it.

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