BEIRUT — Syrian soldiers fought rebels Tuesday in a firefight that killed nine people and sent several mortars sailing across the border into the Israeli-occupied Golan Heights.
The Israeli military said nobody was hurt in the shelling and that the spillover was thought to be accidental.
But Israel filed a complaint to the U.N. peacekeeping force that patrols the tense region between Israel and Syria.
Over the course of the 18-month uprising against Syrian President Bashar Assad, violence has spilled into neighboring Lebanon, Jordan and Turkey. In July, mortar shells fell about half a mile from the Golan boundary.
The spillover is among the most worrying developments from the Syria crisis, which has the potential to inflame the entire region.
Activists said Tuesday that the clashes between troops and rebels inside Syria killed at least nine people.
On Tuesday, U.N. Secretary-General Ban Ki-moon demanded international action to stop the war in Syria, telling a somber gathering of world leaders that the conflict has become "a regional calamity with global ramifications."
"The international community should not look the other way as violence spirals out of control," Mr. Ban said.
An Israeli defense official said the military believes Tuesday's incident in the Golan Heights was a mistake and the mortars were not aimed at the Jewish state.
It was not the first time shells from Syria exploded in Israel since the uprising began, the official said on condition of anonymity because he is not authorized to brief the media.
There have been concerns in Israel that the long-quiet Israel-Syria frontier area could become a new Islamist front against the Jewish state. Israel captured the Golan Heights from Syria in the 1967 Mideast war and annexed the plateau 14 years later.
Syria and Israel are bitter enemies, and have fought several wars, including the 1973 war. Despite the animosity, the border with Syria has been Israel's quietest since then.
The defense official said Israel is concerned that the border region could become as lawless and deadly as Israel's frontier with Egypt's Sinai Peninsula has become since the fall of longtime Egyptian President Hosni Mubarak last year.
Meanwhile, several bombs exploded Tuesday inside a school in the Syrian capital that activists say was being used by regime forces as a security headquarters. Ambulances rushed to the area and an initial report on state media said seven people were wounded.
Over the past few months, rebels increasingly have targeted security sites and symbols of regime power, particularly in the main cities of Damascus and Aleppo, in a bid to turn the tide.