With thousands of troops and tanks massed at the frontier, Israeli air strikes earlier killed a Hamas military commander and destroyed a school as the military campaign against rockets fired from Gaza entered a second week.
Israeli radio warned the public that rocket attacks could intensify in "coming hours".
Israel has staged more than 750 air raids against Hamas leaders and military targets since launching "Operation Cast Lead" on December 27. At least 442 Palestinians have been killed -- including 75 children -- and 2,290 wounded, according to Gaza medics.
Four people have been killed in Israeli by more than 500 rockets fired from Gaza over the same time.
Exiled Hamas leader Khaled Meshaal warned Israel on Friday of a "black destiny" if it invaded. But US leaders have given their key Middle East ally free rein to begin a ground operation, again blaming Hamas for the new conflict.
More than 30 air raids on Saturday hit Hamas targets across the densely populated territory.
One strike killed Mohammad al-Jammal, who Gaza sources said was a Hamas military commander. Israel said he was responsible "for the entire rocket launching enterprise in all of Gaza City."
Jammal's death came two days after an air raid killed top Hamas leader Nizar Rayyan.
Another raid demolished a school in northern Gaza and killed a guard there. Israel said its warplanes had targeted "a college used as a base for firing a large number of rockets."
Two people were killed when a strike hit their car in the southern city of Khan Yunis, medics said.
The Israeli strikes have so far failed in their declared aim of ending rocket fire from Gaza and there is mounting concern over the humanitarian impact of the Israeli operation.
Maxwell Gaylard, UN humanitarian coordinator for the Palestinian territories, said on Friday "there is a critical emergency in the Gaza Strip right now. By any definition this is a humanitarian crisis and more."
About 80 percent of the 1.5 million population relies on international food aid.
But the United States has given fresh backing to Israel, insisting that the key to a truce is Israel's demand that Hamas stop firing rockets.
In his weekly Saturday radio address, the text of which was released by the White House, President George W. Bush called on Hamas "to turn away from terror, and to support legitimate Palestinian leaders working for peace."
Bush blamed Hamas for the violence and rejected calls for a unilateral ceasefire that he said would allow the Islamists to continue targeting Israel with rocket and mortar fire.
Thousands of Israeli troops with tanks have been waiting along the 60-kilometre (37-mile) border with Gaza for the green light from the government to advance.
Amid new diplomatic efforts to halt the fighting, Palestinian president Mahmud Abbas was to meet French President counterpart Nicolas Sarkozy in Ramallah on Monday. He was then to travel to New York to appeal for a ceasefire at the UN Security Council.
Syrian President Bashar al-Assad on Saturday met Iran's Supreme National Security Council chief Saeed Jalili to discuss the Gaza crisis, Syria's official SANA news agency reported.
Jalili also met the exiled Hamas leader Khaled Meshaal and Islamic Jihad leader Ramadan Abdullah Shallah on Friday, a Palestinian source said.
In a televised speech on Friday night, Meshaal warned Israel: "If you commit the stupidity of launching a ground offensive then a black destiny awaits you."
The week of Israeli strikes has destroyed Hamas government buildings, the homes of senior Islamist officials, mosques, schools and other buildings said to have stored weapons, and roads and tunnels used to smuggle arms and supplies.
Israel has kept the territory virtually sealed since Hamas seized power there in June 2007 from Fatah forces loyal to the secular Abbas.
Image of smoke rising following an Israeli air strike in Gaza City, by Mahmud Hams.