The man was working near the Karni commercial crossing when he was shot and seriously wounded, Israeli rescue services said.
"This is just a ploy to regain international hand-outs with smoke and mirrors"
LeotheIsaurian, Detroit, US
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"The Qassam Brigades announced its responsibility for shooting a Zionist [Israeli] and firing two mortar bombs against a gathering of Zionist soldiers near Karni crossing," a statement from the Palestinian Izz al-Din al-Qassam group said.
"Our strikes against the enemy will continue."
The attack comes two days after the Palestinians formed a government of national unity that the Israelis have refused to co-operate with.
An official in the office of Ehud Olmert, the Israeli prime minister, called the shooting a "terror" attack.
"This is precisely the type of terror that the new Palestinian government steadfastly refuses to condemn, thus rejecting a principle condition placed upon it by the international community," the official told Reuters news agency.
It is the first such attack claimed by the armed wing of Hamas since a truce was called between the two sides in November.
Other Palestinian groups, such as Islamic Jihad, have continued to fire makeshift rockets into Israel from Gaza throughout the ceasefire.