JERUSALEM, Dec 16 (Reuters) - An Israeli warplane on Thursday shot down an apparently unmanned balloon that flew over Israel's Dimona nuclear reactor, a security official said.

It definitely flew over Dimona, although we are still trying to determine what that entailed and the military is now handling the matter, the official said.

A military spokeswoman said an Israeli warplane shot down a suspicious flying object, probably a balloon, in southern Israel. Israeli media reports said the balloon was unmanned but powered by an engine.

Airspace over the reactor, where Israel is widely believed to have built more than 200 nuclear warheads, is a no-fly zone.

Last October, Israeli warplanes interceptd an Israeli ultralight aircraft that accidentally flew into the area and forced it to land at an airstrip in southern Israel.

During the 1967 Middle East war, an Israeli surface-to-air missile downed a crippled Israeli fighter-bomber that strayed into the restricted zone. Its pilot was killed. (Writing by Ari Rabinovitch, Editing by Jeffrey Heller)