Gunfire, blasts heard as Israeli troops clash with Hamas fighters in Gaza
UN chief Ban calls for Israel to cease ground operation
Journalists and witnesses at the border reported streaking bullet flashes and balls of fire in the sky over Gaza, while in Gaza City, the territory's largest city, high-rise buildings shook from shockwaves of larger explosions.
The Israeli military said its forces have killed dozens of masked Hamas fighters, while putting its own casualty figures at 30 wounded, two seriously.
Hamas said its gunmen have killed some Israeli soldiers, while Reuters quoted Palestinian medical officials as saying eight Palestinians, five of them gunmen, were killed.
Each side was quick to dismiss the other's claim of causing high casualties as propaganda.
Hours earlier, Israeli Defence Minister Ehud Barak warned the offensive "will not be easy or short."
The incursion is aimed at bringing about a "significant change" in southern Israel's security situation as soldiers attempt to seize some areas that have been used as rocket launch sites, Barak told a news conference in Tel Aviv.
"We are not war-hungry, but we shall not, I repeat, we shall not allow a situation where our towns, villages and civilians are constantly targeted by Hamas," Barak told reporters.
He also said Israel would continue to do "everything possible" to provide humanitarian aid in Gaza where needed.
About 10,000 Israeli troops had been massing at the border with Gaza for several days before the go-ahead to enter the coastal territory, which has been controlled by the Islamist militant group since June 2007.
Maj. Avital Leibovich of the Israel Defence Forces said troops have been training for 2½ years to conduct battles in urban areas.
She said they will implement what they learned in the 34-day conflict with Hezbollah in Lebanon and those soldiers entering Gaza are "very well prepared and trained."
Hamas spokesman Ismail Radwan, however, predicted victory for the militants.
"Gaza will not be a picnic. Gaza will be a graveyard for you," he said in a message to Israeli soldiers.
More than 450 Palestinians have been killed in the offensive, according to Palestinian medical officials. Meanwhile, four Israelis have died in recent Hamas rocket attacks.
UN officials say at least 25 per cent of the Palestinian casualties have been civilians and a serious humanitarian crisis has developed.
No statement agreement at UN Security Council
The United Nations Security Council held a special meeting Saturday evening on the crisis, while UN Secretary-General Ban Ki-moon called for an immediate end to Israel's ground operation.
The UN press office said in a statement that Ban conveyed his "extreme concerns and disappointment" in a telephone conversation Saturday with Israeli Prime Minister Ehud Olmert.
Ban also asked that Israel ensure the safety of civilians and allow humanitarian assistance to reach those in need, according to the statement.
Arab nations demanded that the Security Council call for an immediate ceasefire. But late in the evening, the United States blocked a draft statement circulated by Libya to council members expressing serious concern at the escalation of the situation and calling for Israel and Hamas to halt all military activities.
U.S. deputy ambassador Alejandro Wolff said the United States saw no prospect of Hamas abiding by last week's council call for an immediate end to the violence. Therefore, he said, a new statement now "would not be adhered to and would have no underpinning for success, would not do credit to the council."
Presidential statements become part of the council's official record but press statements are weaker and do not.
In a statement ahead of the UN meeting, the U.S. State Department said the United States is working toward a ceasefire for Gaza that would not allow Hamas to continue firing rockets into Israel.
State Department spokesman Sean McCormack said in the statement that Washington had also told Israel's government that military action must be "mindful of the potential consequences to civilians."
Second Hamas leader killed
News of the latest offensive against Hamas militants came hours after the political group confirmed that Israel had killed one of its senior commanders, the second of its leaders to die in the past week.
Hamas said Abu Zakaria al-Jamal died from wounds sustained in an Israeli air strike overnight. Earlier in the week, another Hamas leader, Nizar Rayyan, was killed along with several family members when the Israeli air force bombed his house.
Israeli warplanes and gunboats struck more than two dozen Hamas targets Saturday, including weapons storage facilities and training centres.
A Palestinian medical official said 14 people were killed and 33 wounded when a bomb hit a mosque in Beit Lahiya, while four others were reported killed in a separate air strike in Rafah earlier in the day.
The Israeli military has said Hamas uses mosques for rocket storage, citing video footage of a previous air strike that show secondary explosions from the buildings.
Meanwhile, Palestinian militants fired at least 40 rockets into southern Israel on Saturday, lightly wounding three people in two separate attacks on Netivot and Ashdod.
Robert Serry, the UN Special Co-ordinator for the Middle East Peace Process, said Hamas rockets are reaching 40 kilometres into Israel.
The Israeli army has dropped thousands of leaflets over Gaza, warning people to leave their homes before targeted bombing raids.
French President Nicolas Sarkozy will visit the region starting Monday and President George W. Bush and UN chief Ban have both spoken in favour of an internationally monitored truce.
In his weekly radio address, Bush blamed the fighting in Gaza on Hamas, accusing militants of waging a campaign of violence against Israel with little regard for Palestinian civilians.
"In response to these attacks on their people, the leaders of Israel have launched military operations on Hamas positions in Gaza," Bush said. "As a part of their strategy, Hamas terrorists often hide within the civilian population, which puts innocent Palestinians at risk."
Chris Gunness, spokesman for the UN Relief and Works Agency, said damage to Gaza's infrastructure is creating a humanitarian crisis.
"Life has become intolerable for civilians," he told CBC News. "This mad bombing has simply got to stop. It's as simple as that."
About 1.5 million Palestinians in Gaza are living under a blockade by neighbouring Israel and Egypt, curbing supplies of fuel needed to run generators.
The Israeli air strikes have further crippled the economy, although Israel had increased its humanitarian aid to the territory.
With files from the Associated Press