GRABOVO, Ukraine — A blast from a shoulder-fired rocket takes down a Ukrainian military transport plane on its landing approach, killing 49 people. Twenty-three Ukrainian soldiers are slaughtered by Russian-made Grad rockets. An AN-26 military plane is shot down at 21,000 feet, an altitude reachable only by a sophisticated surface-to-air missile with radar guidance.

Pro-Russian rebel commanders have made no secret of their pride in carrying out increasingly devastating attacks over the past month, using more advanced weaponry that the United States and its allies say has been provided by Russia. The insurgent leaders have bragged of their successful strikes in interviews and in social media posts — often with links to videos, like one of the AN-26 disintegrating into a streak of white smoke as it fell through the sky.

On Thursday, Ukrainian officials say, that pattern repeated itself — for a time. According to the Ukrainian government, the well-known Russian commander Igor Strelkov — or someone writing under his name — boasted on the Russian version of Facebook about what he thought was the downing of another AN-26. Instead, it turned out to be Malaysia Airlines Flight 17, a civilian jetliner carrying 298 people, including 80 children.

By Friday the rebels were taking an entirely different tack. Mr. Strelkov’s post had been taken down, and leaders were scoffing at the idea they had weapons with such reach.