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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.
Deadly fire broke out on Tuesday, killing both Lebanese and Israeli soldiers.
It's the most serious violence along the frontier since the 2006 war.
Fighting broke out after Israeli soldiers cut down a tree along the border fence.
A journalist says Lebanese soldiers fired into the air at first, but the Israelis responded with deadly aim.
At least five people, including two Lebanese soldiers and an Israeli army officer, were killed.
Israel says the Lebanese army is entirely responsible.
Lt. Col. Avital Leibovich, Israeli Military Spokeswoman, said, "IDF soldiers were on the Israeli side of the border conducting some routine work on the Israeli side of the border where sniper fire - which we assume was well planned - was opened at these soldiers."
Lebanon places full blame on the Israeli side.
Said Eid, Chief of Lebanese Supreme Defense Council, said, "The instructions were given to confront any aggression on our land, our people, our army, in all available means whatever the sacrifices will be."
Lebanese President Michel Suleiman says the shelling was a violation of the UN resolution that ended fighting between Israel and Hezbollah in 2006.
He says Israeli troops crossed the UN-drawn Blue Line boundary and fired on a Lebanese army checkpoint.