The White House on Monday said Hamas must stop firing rockets into Israel and agree to a lasting ceasefire.
Israeli air strikes on Gaza have continued into a third day. They started on Saturday morning, 27 December.
Targets included the Islamic University and the interior ministry on Monday. Government officials and tunnels into Egypt have also been hit.
Palestinian medical officials put the death toll at 313 with more than 1400 wounded.
The attacks are in reliation for rocket attacks on Israel which intensifed after Hamas ended a ceasefire on 19 December.
The United States has placed the onus for ending the violence on Hamas which Washington considers a terrorist organization.
An election is due to be held in Israel on 10 February.
All-out war
Israeli defence minister Ehud Barak says Israel is involved in an "all-out war" against Hamas and its leadership.
Mr Barak told the Israeli parliament on Monday that the goal of the current military operation was to stop Hamas attacking Israeli civilians and soldiers - a reference to Palestinian rocket fire from Gaza.
Mr Barak said the military is doing all it can to avoid civilian casualties.
The BBC reports Israel has massed forces along the border and has declared the area around the narrow coastal strip a "closed military zone".
Israel withdrew from the the territory in 2005 but has kept tight control over access in and out of Gaza and its airspace.
"Dark moment"
British Foreign Secretary David Miliband has warned of a very dark moment in the Middle East peace process as the Israeli air strikes continue.
Mr Miliband says events threaten attempts to build a comprehensive peace and to fuel radicalism.
Calling for an urgent ceasefire, he says a terrible price is being paid for faltering peace negotiations.
