WASHINGTON — The first vivid demonstration of the war in Ukraine’s sudden escalation came Monday, three days before the downing of a Malaysian civilian airliner.

A Ukrainian AN-26 transport plane was shot down by a surface-to-air missile while flying at 21,000 feet, an attack that American and Western officials believe was carried out either by Ukrainian separatists allied with Moscow or possibly even a Russian military unit.

More than half a dozen aircraft had been shot down in Ukraine in recent months, but this attack was different. Before Monday, only aircraft flying at relatively low altitudes — within the range of shoulder-fired weapons — were thought to be vulnerable to ground fire.

For the first time, a surface-to-air missile with greater range had been used, raising questions about whether the rebels have acquired such a devastating capability, and Russia’s role.