Hamas said the killing of al-Jaabari was "a declaration of war. "
Israel "committed a dangerous crime and broke all redlines," said Fawzi Barhoum, a spokesman for Hamas. "The Israeli occupation will regret and pay a high price."
Meanwhile, Palestinian President Mahmoud Abbas called for an urgent meeting of the Arab League (AL) to discuss the escalation in Gaza and prevent a large-scale Israeli operation.
The surge in violence broke a day of relative calmness that prevailed in the area after days of cross-border exchange of fire between Israel and Palestinian militant groups.
Al-Jaabari is the highest-ranking Hamas member to be killed by Israel since the end of Operation Cast Lead, a three-week Israeli military operation in Gaza that saw the death of as many as 1,400 Palestinians in December 2008 and January 2009.
The death of al-Jaabari, 52, is highly expected to set off a fierce surge in violence in which Hamas is likely to use a large portion of its military capacity in attacking Israeli targets, possibly by using longer-range rockets.
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