Suspicion fell on militant group Lebanese Islamic Hizbollah after an unmanned surveillance drone that flew into southern Israel was shot down by Israeli jets.
The drone crossed into the country's airspace from the Mediterranean, crossing the Palestinian territory of Gaza before being intercepted.
Although initial reports from a Palestinian news agency said the drone had come from Gaza, Israel denied this, adding that its aircraft had encountered it over Israeli territorial waters.
The source of the drone is a mystery but there have been two instances of Hizbollah-owned drones entering Israeli airspace in the past.
It is known that, in this incident, the drone was in flight over Israel for less than 30 minutes and was not carrying any explosives.
In the 2006 Israel-Lebanon war, Hizbollah launched two Iranian-made Ababil drones capable of carrying explosives over Israel. Both were shot down: one over the sea in Lebanese airspace, the other north of Haifa. Two years earlier, another drone crashed in the Mediterranean.
This year Lebanese security sources reported two incidents in Lebanon involving unmanned aircraft belonging to Hizbollah.
Israel recently gave Egypt permission to fly drones along the two countries' shared border as part of its efforts against armed groups operating in the Sinai.
An Israeli military spokeswoman, Lieutenant-Colonel Avital Leibovich, said Israeli ground defence systems had alerted the air force to the drone's presence.
"We alerted jet planes that escorted the unmanned aerial vehicle and due to some operational considerations we decided to intercept the unmanned aerial vehicle in the northern Negev area, which has no population."
Defence Minister Ehud Barak said that Israel views "this incident of attempting to enter Israeli airspace very severely and we will consider our response later", while Prime Minister Benjamin Netanyahu vowed that Israel "will continue to defend our borders in the sea, on the land and in the air for the security of the citizens of Israel".
Observer, APBy Peter Beaumont