By Maayan Lubell
JERUSALEM (Reuters) - Israel shot down a Syrian warplane on Tuesday, saying the aircraft crossed the battle lines of Syria's civil war and flew over the Israeli-held Golan Heights, perhaps by accident.
The incident coincided with but did not appear to be directly related to air strikes the United States and Gulf Arab allies mounted on Islamic State strongholds in Syria.
But it presented another challenge to Israel's oft-stated desire to stay on the sidelines of a conflict on its northern doorstep, in which al Qaeda-linked Nusra Front rebels took over a border crossing on the Golan last month.
The Israeli military said its U.S.-made Patriot missile air defence system shot down a Syrian Russian-built Sukhoi fighter plane that had "infiltrated Israeli airspace" over the territory, captured by Israel in the 1967 Middle East war.
It was the first time in three decades that Israel had downed a Syrian warplane.
The Syrian Observatory for Human Rights, which tracks violence in the civil war, said the warplane had been bombing areas outside Quneitra, a Syrian town near the Israeli-held side of the frontier, at the time it was shot down. It said the pilot had bailed out.
Syria described the downing of the aircraft as an act of aggression.
Israeli Defence Minister Moshe Yaalon, commenting on the incident, said Israel "will not allow any element - neither a state nor a terrorist group - to threaten our security and violate our sovereignty".
Israeli military sources said the plane apparently crossed by accident into Israeli-controlled airspace.
In his statement, Yaalon also seemed to raise that possibility, saying Israel would respond strongly to perceived threats "whether they stemmed from a mistake or were deliberate".
Israel has fired into Syrian territory on numerous occasions in response to shelling on the Golan that the military has said was largely spillover from fighting between rebels and the Syrian Army.
An Israeli Patriot missile destroyed a Syrian drone over the Golan on Aug. 31. Israel last downed a manned Syrian aircraft in 1985, when Israeli fighters on a surveillance mission over Lebanon destroyed two Syrian MiG-23s that approached them.
(Writing by Jeffrey Heller; Additional reporting by Tom Perry; editing by Janet McBride)