BEIRUT, Lebanon — Syrian rebels downed a military helicopter with a surface-to-air missile outside Aleppo on Tuesday, video uploaded by antigovernment activists appeared to show, marking what is potentially a major battlefield advance: confirmation that rebels have put their growing stock of heat-seeking missiles to effective use.

In one video, a utility helicopter that appeared to be a Russian-built Mi-8 can be seen banking in a slow left turn and then being hit squarely near its engine by a fast-moving projectile rising at a sharp angle from below. Another video showed what appeared to be the same helicopter moments after the strike. The crippled aircraft manages a partly controlled descent in spreading flames, as a voice off-camera shouts, “sarook,” or rocket, before it strikes the ground and explodes.

In recent months, rebels have used mainly machine guns to shoot down several Syrian Air Force helicopters and fixed-wing attack jets. In this case the thick smoke trailing the projectile, combined with the elevation of the aircraft, strongly suggested that the helicopter was hit by a missile.

Rebels hailed the event as the culmination of their long pursuit of effective antiaircraft weapons, though it was not clear if the downing was an isolated tactical success or heralded a new phase in the war that would present a meaningful challenge to the Syrian government’s air supremacy.