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Experts say a surface-to-air missile with a mid-range would be well able to strike a plane that was reportedly flying at an altitude of more than 30,000 feet (10 kilometres).
Both Russian and Ukrainian forces have variants of the Buk, a surface-to-air missile system, including SA-11 and SA-17 missiles, that can hit targets at an altitude of up to 25 kilometres.
A post on the official Twitter account of the "Donetsk People's Republic" and later removed, claimed that the separatists early on Thursday had seized a Buk missile system from the Ukrainian army, capable of downing a jet at that altitude.
Under NATO's terminology, the Buk missile is referred to as the "Gadfly."
Shoulder-launched weapons have been blamed for the downing of several Ukrainian aircraft in recent days but those attacks occurred at much lower altitudes, analysts said.
"A short-range, shoulder-launched weapon has been responsible for several aircraft lost in the last few days. . . but it wouldn't be able to reach an altitude of 30-odd thousand feet," said Edward Hunt, senior defence analyst at IHS Jane's consultancy.
"It would have to be a missile of a certain capability," Hunt told AFP.
Another scenario for the downed airliner could involve an air-to-air missile launched from a fighter jet, though there has been no indication of a warplane nearby at the same time.
The Buk surface-to-air missiles are in wide use. Before the conflict in Ukraine erupted Kiev government forces had about six to eight batteries, Hunt said.
The Buk systems have appeared on Red Square at military parades and were first produced in the 1970s during the Soviet era.
The latest versions are manufactured at a factory in Ulyanovsk by Almaz-Antey, a firm that has been targeted by recent US sanctions against Moscow.
The Buk missiles also reportedly have been spotted in Syria. Russia delivered the systems to Damascus in recent years.
