According to the Pentagon, U.S. and 俄国n combat planes have had some tense moments over 叙利亚.
那里 have been several close encounters between 俄国n and U.S. and allied manned and unmanned aircraft over 叙利亚 since Moscow deployed a contingent of combat planes to 拉塔基亚, in northwestern 叙利亚, at the end of 2015年9月.
According to 俄国’s Ministry of Defense, the 侧卫衍生4 ++ Gen飞机 was providing air escort for a package of attack planes in 叙利亚 when it approached the American plane, to perform a VID (Visual Identification) even though some sources suggested it was the 俄国n aircraft to be intercepted by a U.S. plane.
其中一个事件涉及 来自土耳其Incirlik的美国F-16和一些RuAF Su-34。
After the first incidents, 俄国 and the US agreed on coordinating their air activity in the skies over 叙利亚. More or less.
甚至在之后，仍经常报告亲密接触和挑衅 Nov. 24, 2015 when a Turkish Air Force (TuAF) F-16 shot down a 俄国n Air Force (RuAF) Su-24 据称侵犯了土耳其领空。
四架以色列F-15与两架F-15之间的“近冲突” 俄国n Su-30SM据称是4月20日发生的，距普京下令 withdrawal of part of the 俄国n combat planes from Hmeymim空军基地: flying over the Mediterranean Sea, the Israeli jets approached 拉塔基亚 forcing the 俄国ns to scramble two of their Sukhoi jets. 以色列的几家媒体 甚至说 俄国ns 向以色列军机发射了至少两次，但从未确认任何事件。
确实，正如 有线电视新闻网, U.S. F/A-18s were somehow close to engage 俄国n Sukhois (still not clear whether Su-34s or Su-24s as there are conflicting reports on the type of aircraft involved) that bombed U.S.-backed 叙利亚n rebels near the Jordan border.
这里’根据西奥多·施莱弗（Theodore Schleifer）和芭芭拉·斯塔尔（Barbara Starr）的看法：
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 俄国ns 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 俄国ns came back for another round of bombing, the defense official said.
“俄罗斯飞机在坦夫附近对叙利亚反ISIL部队进行了一系列空袭，其中包括得到美国支持的人员。俄罗斯飞机在叙利亚南部这一地区已经有一段时间没有活动了，附近没有叙利亚政权或俄罗斯地面部队，”一位高级国防官员说。“Russia’s latest actions raise serious concern about 俄国n intentions. We will seek an explanation from 俄国 on why it took this action and assurances this will not happen again.”
The first two bombing runs by the 俄国ns were carried out by two SU-24 俄国n jets coming out of their base near 拉塔基亚. 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.”
So, it looks like the American Hornets were pretty close to intercepting the Sukhois (in other reports they were able to visually ID the 俄国ns), tried to contact the 俄国n planes as these carried out an air strike, but these simply ignored the calls on a previously agreed radio frequency.
The question is what are the ROE (Rules Of Engagement) in place over 叙利亚? Most probably there are strict ROE to prevent escalation and avoid direct confrontation but what would have happened if the U.S. F/A-18 had intercepted the 俄国n warplanes attacking the US-backed rebels ignoring the American calls?