Top Israeli officials have vowed to continue attacks on Hamas, as Israeli air strikes pounded the Gaza Strip for a third day.
Defence minister Ehud Barak says Israel is involved in an "all-out war" against Hamas and its leadership.
He told the Israeli parliament on Monday that the goal of the current operation was to stop Hamas attacking Israeli civilians and soldiers - a reference to Palestinian rocket fire from Gaza.
A top army official said no Hamas buildings would be left standing.
The UN says about 320 Palestinians have died since Saturday. Four Israelis have been killed by rockets from Gaza.
The Red Cross describes the situation in Gaza's hospitals as chaotic, with medical teams "stretched to the limit".
The BBC reports dozens of Hamas agencies, including security compounds, government offices and tunnels into Egypt, have been hit since Israel began the bombing on Saturday morning.
Early on Monday, raids damaged both the interior ministry and a science building at the Islamic University in Gaza.
Places hit by later strikes included the home of a senior Hamas commander.
In his third statement in three days, U.N Secretary-General Ban Ki-moon again called for an immediate ceasefire by both sides.
Mr Ban said on Monday he was "deeply alarmed" by the escalation of violence in Gaza. While recognising Israel's right to defend itself from rocket attacks, he condemned its "excessive use of force".
Boundary build-up
Israel has massed ground forces along the boundary with Gaza and has declared the area around it a "closed military zone".
The bombings are in retaliation for continuing rocket attacks which have intensified since Hamas ended a truce on 19 December.
An election is due to be held in Israel on 10 February.
