In a new twist between Russia and the US for the ongoing control of the campaign against ISIL in Syria, Russian warplanes bombed U.S. backed Syrian rebels near the Jordanian border on Friday. Pentagon officials say the Russian intervention from two SU-34 Fullbacks caused the U.S. to divert armed aircraft to the scene of the strike.

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Moscow backs the Assad dictatorship; the U.S. backs armed rebels who have been trying to overthrow Assad’s regime. The area attacked was a “staging area” camp for counter ISIL forces.

“Russian aircraft conducted a series of airstrikes near al-Tanf against Syrian counter-ISIL forces that included individuals who have received U.S. support. Russian aircraft have not been active in this area of Southern Syria for some time, and there were no Syrian regime or Russian ground forces in the vicinity,” a senior defense official said.

Secretary of Defense Ash Carter was known to be livid about the attack in private, but more reserved in his public statement.

“This was an attack on forces fighting ISIL and that is problematic,” Carter said, adding that faulty intelligence might be to blame.

Carter also noted: “Here’s a case where they actually attacked forces that were fighting ISIL. And if that was their intention, that’s the opposite of what they said they were going to do,” leaving many to query if this is simply another sabre rattling moment for Putin to exploit.

Russian SU-34’s Escalate Tensions between Moscow and US

Read Next: Russian SU-34’s Escalate Tensions between Moscow and US

Russian President Vladimir Putin heads the Cabinet meeting in the Novo-Ogaryovo residence, outside Moscow, Russia, Wednesday, July 30, 2014. The meeting focused on measures to encourage Russian companies to pull their assets back from offshores. The United States and the European Union on Tuesday announced a raft of new sanctions against Russian companies and banks over Moscow’s support for separatists in Ukraine. (AP Photo/RIA Novosti, Alexei Nikolsky, Presidential Press Service)

A section of armed FA-18s was diverted to the area after the first round of two strikes in an effort to set up a top cover for the armed rebels. Aircrew from a nearby E-2 Hawkeye also tried to call the Russians on a previously agreed-upon pilot-to-pilot communications channel but did not receive an answer. Reports are that when the FA-18’s temporarily left the area to refuel, the SU-34’s returned and re-attacked those same targets leaving a small but unknown number of rebel forces dead.

US fighters follow a strict set of rules, more commonly referred to as “rules of engagement” or ROE. While the rules of engagement are classified, the assumption based off what occurred Friday is that all means of communication are used to the fullest extent prior to engaging any enemy fighters. It is not known what Russia’s rules of engagement are, but it is clear that Putin has ratcheted up the tension between the two sides with this incident.

The US and Russia have been operating in the same airspace since Russia began its Syria campaign in September of 2015. The two powers have used a communication hotline to de-conflict airspace but since Russian aircraft had not been recently active in the area, the bombing came as a surprise to US officials.

Surprises are never good in aviation and war. With an unknown quantity like Russia, the US must be extremely vigilant or risk an unnecessary and unwanted escalation.