Israel killed a Hamas strongman yesterday by dropping a one-tonne bomb on his house, also killing two of his wives and four of his children.
This first assault on the top leadership of Gaza's ruling group escalated a crushing aerial offensive as Israel declared itself ready to launch a ground invasion.
"We are trying to hit everybody who is a leader of the organisation, and today we hit one of their leaders," said Israeli vice-prime minister Haim Ramon.
Medics say a total of 13 members of Rayyan's family were killed by the airstrike on the residential neighbourhood.
A Hamas official said: "The blood of Sheikh Nizar Rayyan and the blood of other martyrs will never be wasted and the enemy will pay a heavy price for the crimes it has committed."
Thousands of soldiers were massed along the border with Gaza yesterday, backed by tanks and artillery.
While turning up the heat on the battlefield, Israel appeared to be sounding out a possible diplomatic exit from its campaign, demanding international monitors as a key term of any future truce.
In mounting the campaign last Saturday, Israel made it clear no one in Hamas was immune, and the air attack that flattened the four-storey apartment building in the northern town of Jebaliya where Nizar Rayan lived drove that message home.
Hamas leaders went into hiding before Israel launched its operation, but Rayan was known for openly defying Israel, and the army said he had a tunnel under his house that could serve as an escape route. Rayan (49) ranked among Hamas' top five decision-makers.
A professor of Islamic law, he was known for his close ties to the group's military wing and was respected in Gaza for personally participating in clashes against Israeli forces.
At least 420 people have been killed and 2,100 injured since Israel's aerial bombardment of the Gaza strip began six days ago.