A column of Israeli tanks, backed by aircraft, pushed deep into the territory, and Israel's navy prevented travel along Gaza's coastal road, effectively cutting the enclave in half.

In initial fighting, Israeli ground forces killed eight Gazans, five of them gunmen, bringing the Palestinian death toll since the start of an air campaign on December 27 to more than 450, Palestinian medical officials said.

Israel said 30 of its soldiers were wounded, two seriously, since the start of the ground assault and that Israeli aircraft struck more than 45 targets, including arms smuggling tunnels, weapons depots and mortar squads.

At the United Nations, the United States thwarted an effort by Libya to persuade the Security Council to call for an immediate ceasefire.

Israel said it called up tens of thousands of reservists and the military's chief spokesman estimated the operation in the Hamas-run territory could take "many long days."

Heavy casualties are likely to increase international pressure on Israel to halt its biggest operation in the Gaza Strip in four decades, fighting that holds significant political risks for Israeli leaders ahead of a February 10 national election.

The plight of the 1.5 million Palestinians crammed into the Gaza Strip was growing more desperate. People have taken shelter in their homes for days and humanitarian agencies warned that water, food and medical supplies were running short.