By Nidal al-Mughrabi
GAZA (Reuters) - Israeli tanks and infantry battled Hamas fighters in the Gaza Strip on Sunday in a ground offensive launched after eight days of deadly air strikes failed to halt the Islamist group's rocket attacks on Israel.
Wearing night-vision goggles on their helmets and camouflage paint on their faces, Israeli soldiers entered the densely populated enclave on Saturday evening with tank columns sweeping in from four points and combat helicopters flying overhead.
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.
"During exchanges of fire overnight, dozens of armed Hamas operatives were hit," an Israeli military communique said.
At the United Nations, the United States thwarted an effort by Libya to persuade the Security Council to call for an immediate cease-fire, diplomats said.
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.
The Israeli military said "large infantry, tank, engineering, artillery and intelligence forces" were operating throughout the Gaza Strip, backed by attacks by aircraft and warships off the Mediterranean coast.
A spokesman for Hamas's armed wing, the Izz el-Deen al-Qassam Brigades, said Israeli troops faced certain death or capture. "The Zionist enemy must know his battle in Gaza is a losing one," said the spokesman, Abu Ubaida.
At least a quarter of the 453 Palestinians killed in the current conflict have been civilians, a U.N. agency said. Another 2,050 Palestinians have been wounded. A leading Palestinian rights group put the number at 40 percent.
Four Israelis have been killed by rockets that continue to pound southern Israel.
In New York, the U.N. Security Council held a special meeting to discuss the latest developments. Several council diplomats said the U.S. refusal to back the Libyan-drafted demand for an immediate truce had killed the initiative, since council statements must be passed unanimously.
The U.S. State Department said a cease-fire should take place "as soon as possible," in a statement that urged Israel to be "mindful of the potential consequences to civilians" but did not refer directly to the invasion or call for an immediate truce.
Washington, the statement said, was working towards a cease-fire that would not allow for a re-establishment of the status quo, "where Hamas can continue to launch rockets out of Gaza and to condemn the people of Gaza to a life of misery."
A spokesman for Palestinian President Mahmoud Abbas condemned the Israeli attack as "a vicious aggression."
Israeli Defence Minister Ehud Barak said the aim of the ground push was to "protect the home front" from rocket attacks. He refrained in a televised address from making any threat to try to topple Gaza's Hamas government.
"It won't be easy. It won't be short," said Barak, leader of the centre-left Labour party and a candidate for prime minister in an election that opinion polls predict will return right-winger Benjamin Netanyahu to power.
Major Avital Leibovich, an Israeli army spokeswoman, said the objective "is to destroy the Hamas terror infrastructure in the area of operations." Israeli commentators said the offensive was also aimed at boosting Israel's deterrence power.
Israeli troops face Hamas fighters whom the United States and Israel say have received arms and training from Iran. Hamas is believed to have about 25,000 fighters and has placed landmines and other traps in anticipation of an invasion.
An Egyptian-brokered six-month truce expired on December 19 but it had been strained by Hamas rocket strikes and Israeli military operations against the group.
Former British Prime Minister Tony Blair, representing major powers sponsoring Middle East peace talks, planned to begin shuttling on Sunday between Israeli leaders and Palestinian leaders -- Hamas's rivals -- in the occupied West Bank.
But divisions within the European Union over the Israeli assault could buy Israel more time. France condemned the Israeli ground assault, as well as Hamas rocket fire. On Monday, French President Nicolas Sarkozy is scheduled to go to Jerusalem.
(Writing by Jeffrey Heller and Adam Entous; Additional reporting by Dan Williams; Editing by Matthew Jones)