"Department officials expressed strong concerns about the attack on the coalition-supported counter-ISIL forces at the At-Tanf garrison, which included forces that are participants in the cessation of hostilities in Syria, and emphasized that those concerns would be addressed through ongoing diplomatic discussions on the cessation of hostilities," Cook said.  "Regarding safety, department officials conveyed that Russia's continued strikes at At-Tanf, even after U.S. attempts to inform Russian forces through proper channels of on-going coalition air support to the counter-ISIL forces, created safety concerns for U.S. and coalition forces," Cook added.  The U.S. had requested Russian responses to address American concerns about the Thursday incident. Russian warplanes bombing of the rebels caused the U.S. to divert armed aircraft to the scene of the strike.  This video conference session was convened under the auspices of the Safety of Flight Memorandum of Understanding, an arrangement between the counter-ISIL coalition and Russian Federation to maintain safety in the air space over Syria.  The strikes, which the U.S. says killed some New Syrian Army troops, occurred about six miles from the Jordanian border, according to a U.S. defense official. The U.S. diverted armed FA-18s to the area after the first round of two strikes, and the pilots then tried to call the Russians on a previously agreed-upon pilot-to-pilot communications channel but did not receive an answer.  As soon as the U.S. jets left the area to refuel, the Russians came back for another round of bombing, the defense official said.  "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. "Russia's latest actions raise serious concern about Russian intentions. We will seek an explanation from Russia on why it took this action and assurances this will not happen again."  The first two bombing runs by the Russians were carried out by two SU-24 Russian jets coming out of their base near Latakia. The jets dropped what is believed to be the equivalent of U.S. 500-pound bombs and possibly cluster munitions, according to the U.S. defense official.  The video conference between American and Russian officials is highly unusual. The communications channel was not established for this sort of purpose.  U.S. officials are watching the situation closely. Asked about the strikes Friday, Defense Secretary Ash Carter said it raised questions about whether the Russians were actually in Syria to fight Islamic extremists.  "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," Carter said. "If not, then it says something about the quality of the information upon which they make airstrikes."  U.S. and Russian forces in Syria have had tense relations since the country devolved into civil war. The U.S. has backed rebel groups while the Russians have supported Syrian president Bashar al-Assad.  CNN's Jamie Crawford contributed to this report.
U.S. and Russian military officials on Saturday held a video conference to discuss recent Russian air strikes that hit U.S.-backed Syrian rebels, Pentagon Press Secretary Peter Cook said in a statement.