Israel has fired on Syrian forces for the first time in nearly four decades after a mortar round from Syria hit an Israeli post in the Golan Heights.
Israel has also responded with air strikes on its southern border following mortar and rocket fire from Gaza.
This is a dramatic illustration of how Israel might be drawn into the Syrian civil war. It is the first time Israeli forces have fired into Syria since the Yom Kippur of 1973. And this was not just rifle shots the Israelis fired, it was a guided anti-tank missile.
They say that it was in warning, and was directed towards a Syrian army mortar crew, who presumably were taking on Syrian rebels and over shot their target, and instead hit an Israeli military post within the occupied Golan Heights.
In the past week or so, there have been stray bullets coming across that border. At one time, three Syrian tanks drove into what should be a demilitarised zone. Israeli's say that they do not think they are being deliberately provoked, that all this is accidental.
They are desperate to stay out of the Syrian conflict, surrounded as they are by a sea of other troubles, particularly tonight on their southern border with Gaza.
This could get more serious. That border, over this weekend alone, has seen more than 80 rockets fired by Palestinian militants. Four Israeli soldiers were injured, six Palestinians were killed.
Neither side are looking for all out conflict though. But it only really takes one stray missile to do a lot of damage to a civilian target, and I think then the Israeli government might find it very hard to hold back.
Prime Minister David Cameron said Syria's President Bashar al Assad should face "full international justice" for his "appalling crimes".
The Prime Minister has faced questions about his promotion of arms deals during a three-day tour of the Gulf region that started today.