President Poroshenko's official website says a search-and-rescue mission is under way in an effort to "deliver the crew of the plane [shot down today] to territory controlled by Ukrainian servicemen." Citing a briefing of the president by Defense Minister Heletey, the statement says the circumstances of the shootdown of the An-26 transport military plane -- including the 6,500-meter altitude of the aircraft -- makes "its damaging by MANPADS impossible. Thus, the plane was downed by another, more powerful missile weapon that probably was used from the territory of the Russian Federation."
The Interpreter says this YouTube video appears to show the shootdown, at around the 24-second mark:
There are believed to have been around 20 crew members aboard the AN-26 when it was shot down.
Meanwhile, Russia said it will allow members from the Organization for Security and Cooperation in Europe (OSCE) to monitor two checkpoints on its border with Ukraine.
Andrei Kelin, Russia's OSCE envoy, made the announcement in Vienna.
Here's the related announcement by the Russian Foreign Ministry.