The heavily fenced Syria-Israel border crossing on Wednesday - little sign here that a cross border attack may have taken place.

But now security sources and a Western diplomat say that's what happened somewhere on the Syrian border with neighbouring Lebanon.

One security source said there was definitely a hit in the border area, while the diplomat said "something has happened" without elaborating, leaving the events -- at least for now -- shrouded in mystery.

The Lebanese army reported a heavy presence of Israeli jet over its territory throughout the night.

Israelis are on alert for any potential attack from Syria or Lebanon following a series of government warnings - some are here on Wednesday collecting gas masks.

Israel's Vice Premier Silvan Shalom said on Sunday that any sign that Syria's grip on its chemical weapons was slipping could trigger Israeli intervention.

Israeli sources said on Tuesday, though, that Syria's advanced conventional weapons would represent as much of a threat to the Jewish state as its chemical arms should they fall into the hands of Syrian rebel forces, or Hezbollah guerillas based in Lebanon.

Lebanon frequently complains that Israeli jets overfly its territory.

However the recent activity was reported to be much more concentrated than usual.