According to this CBS News report, "CBS News Correspondent David Martin reports that U.S. government sources believe that the Russian plan was downed by a surface-to-air missile fired from the Crimea Peninsula by terrorists."

However, the report also states "The plane, a Tupolev 154, went down 114 miles off the Russian coastal city of Adler, located on the Georgian border, said Vasily Yurchuk, a spokesman for the Ministry of Emergency Situations. Garik Ovanisian, the pilot of the Armenian An-24, said his plane was at 20,790 feet above the Black Sea when the plane above his exploded. "I saw the explosion on the plane, which was above me at an altitude of 36,300 feet," Ovanisian said. "The plane fell into the sea, and there was another explosion in the sea. After that I saw a big white spot on the sea and I had the impression that oil was burning."

I know of no surface to air missile with that kind of range. Even if a missile was fired from a ship below the airliner's flight path, it'd have to be substancially larger than a shoulder-launched SAM to reach that altitude. Color me skeptical until more information is available.