Israel says it attacked more than 60 targets overnight in Gaza as its offensive against Hamas entered its 18th day.
The air assault came as Israeli troops advanced in the southern and eastern suburbs of Gaza City.
The Israeli military also announced another three-hour ceasefire, starting at 0900 local time (0700 GMT), to allow aid lorries into Gaza.
The truce coincides with visits by UN and Red Cross officials to Gaza.
On Tuesday, the western areas of Gaza City also came under shellfire from Israeli gunboats.
The Israeli military has denied a Hamas claim that it had destroyed two Israeli tanks.
The BBC's Aleem Maqbool, on the Israeli-Gaza border, said Israeli shelling had continued despite the three-hour humanitarian ceasefire.
The UN secretary general, Ban Ki-moon, has implored Israel and Palestinian militants to halt the fighting in Gaza immediately.
Ahead of a trip to the region to push for a truce, Ban Ki-moon said too many people had died and there had been too much civilian suffering.
"My message is simple, direct and to the point: the fighting must stop," Mr Ban told a news conference in New York ahead of his departure on Tuesday for the Middle East.
"In Gaza, the very foundation of society is being destroyed: people's homes, civic infrastructure, public health facilities and schools."
His diplomatic tour will see talks with the leaders of Egypt, Israel and Syria as well as the Palestinian president in Ramallah.
However, UN officials say he will not be meeting representatives of Hamas, and it is not clear whether he will go to Gaza itself during his week-long trip.
Also on Tuesday, an Israeli army patrol in the West Bank came under fire from inside Jordan, the army said. No-one was hurt in the incident and the patrol returned fire.
Israeli Prime Minister Ehud Olmert said Palestinian militants will keep on feeling Israel's "iron fist" as long as Hamas fires rockets at Israel.
But a senior Hamas leader, Ismail Haniya, said the group was "approaching victory".
"After 17 days of this foolish war, Gaza has not been broken and Gaza will not collapse," he said in a televised address from a secret location in Gaza.
Both Hamas and Israel rejected last week's UN Security Council resolution calling for an immediate ceasefire.
Palestinian medical sources say 910 people have been killed in Gaza so far, of whom 292 were children and 75 were women. Israeli officials say 13 Israelis, including three civilians, have been killed.
On Monday, casualty reports from Palestinian medics ranged from nine to 26 dead, while Israel said five of its soldiers had been injured, one of them seriously.
Israel is preventing international journalists from entering Gaza, making it impossible to independently confirm casualty figures.
Meanwhile, reports suggest diplomatic efforts between Egypt and Hamas in Cairo are progressing.
After meeting Egyptian President Hosni Mubarak, Middle East Quartet envoy Tony Blair said the elements were in place for a ceasefire agreement.
"I am hopeful we can put an agreement together but it's going to have to be worked on very hard and it's got to be credible," he told journalists.
Israel hopes the scale of its operation will greatly reduce the number of missiles fired from Gaza into southern Israel, while eroding support for Hamas.