Ever since Israel's war with the Lebanese-based Hezbollah four years ago, there have been constant warnings from both sides that a return to violence is just around the corner.

Those threats transformed into deadly fire on Tuesday with the killing of Lebanese and Israeli soldiers during an exchange across their shared border.

While the Lebanese army and the Israel Defense Forces (IDF) enjoy quiet cooperation via the United Nations Interim Force in Lebanon (UNIFIL) peacekeeping force, Israel and Hezbollah are firmly eyeing one another in a constant state of alert.

Israel has repeatedly warned, though, since the summer of 2006, that if Hezbollah attacks Israel, then Israel would hold the Lebanese government responsible.

Initial confusion

In the first hours after the initial reports of a cross-border clash, the picture was one of confusion.

Israeli army said it was fired on by people wearing Lebanese military uniforms. "An IDF force was fired upon from within Lebanon. The soldiers were on routine activity in Israeli territory, in an area that lies between the 'blue line' (the internationally recognized border between Israel and Lebanon) and the security fence, thus within Israeli territory," read a statement from the IDF.

However, in Lebanon, the claim was that Israel fired an artillery shell over the frontier. The shell hit a military vehicle, leaving at least two Lebanese soldiers dead.

The Israeli government made its position very clear a short time after the incident. "Israel views the firing from Lebanon on an IDF patrol, which was operating along the Lebanese border in coordination with UNIFIL, as a grave violation of the UN Security Council Resolution 1701," the country's Foreign Ministry said in a statement, adding that it is filing an official protest with the UN.

"Israel holds the Lebanese government responsible for the grave incident, and warns of the consequences should these violations continue," the statement added.

The Lebanese leadership also filed a protest with the UN and roundly condemned Israel's "violation of Lebanese sovereignty and demands... the UN and the international community bear their responsibilities to pressure Israel to stop its aggression," the Lebanese web portal Naharnet quoted the country's Prime Minister Saad Hariri as saying.

Syrian President Bashar al-Assad phoned his Lebanese counterpart Michel Suleiman to express "Syria's standing by Lebanon against the heinous aggression launched by Israel on the Lebanese territories," according to the official Syrian news agency SANA.

After the initial fire, the border region was reported to be calm but tense, with UNIFIL saying it was working to keep the peace.