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Palestinians stand amid the rubble of a building used by the security forces of Hamas at the site of an Israeli missile strike Saturday at the security headquarters in Gaza City. ((Thaer Al-Hasani/Associated Press))

The death toll has risen past 225 following Israeli air strikes Saturday on dozens of Hamas security compounds in the Gaza Strip, while hundreds of Israeli troops are said to be heading to the border in preparation for a possible ground invasion of the Palestinian territory.

More than 400 people have been wounded in the air strikes and 230 killed, Gaza health official Dr. Moaiya Hassanain said late Saturday. Although most of the dead were Hamas security forces, at least 15 were civilians, according to Palestinian officials.

Militants in Hamas-controlled Gaza responded to the wave of Israeli missiles by sending at least 180 rockets and mortar shells crashing into Israeli border communities, Israeli officials said, killing at least one Israeli and injuring six.

Hamas said it would seek revenge for the air strikes, including launching more rocket attacks and sending suicide bombers to Israel.

Meanwhile, hundreds of Israeli soldiers were reportedly moving toward the Gaza border in advance of a possible ground invasion, military officials told the Associated Press on condition of anonymity.

The offensive, which comes after several days of rocket attacks on Israeli towns and communities and a week after a ceasefire between Israel and Gaza militants ended without a renewed agreement, marks the single bloodiest day the area has seen in years.

The air strikes, which have seen 90 tonnes of bombs dropped on the Palestinian territory, began just before noon local time and continued well into the evening. Hamas officials said all of Gaza's security compounds were destroyed. Israel Army Radio said at least 40 targets were hit.

Among the dead was Gaza's police chief, Maj. Gen. Tawfiq Jaber, witnesses said.

The strikes caused widespread panic and confusion as black smoke rose above Gaza City. Sirens wailed through the streets and women frantically looked for their children.

Response to rocket attacks

An Israeli military statement said the operation targeted "Hamas terror operatives" as well as training camps and warehouses storing weapons. Israel had warned in recent days it would strike back hard against more than 300 rocket attacks launched by Gaza militants on Israeli border towns since last Friday.

Israeli Prime Minister Ehud Olmert said the goal of the offensive was "to bring about a fundamental improvement in the security situation of the residents of the southern part of the country."

He added, "It could take some time."

Palestinian Authority leader Mahmoud Abbas in the West Bank said in a statement that he "condemns this aggression," according to his aide, Nabil Abu Rdeneh.

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An injured Palestinian security force officer from Hamas crawls as he waits for assistance at the site of an Israeli missile strike at the security headquarters in Gaza City. ((Thaer Al-Hasani/Associated Press))

Hamas has vowed intense retaliation.

"Hamas will continue the resistance until the last drop of blood," said Hamas spokesman Fawzi Barhoum on a Gaza radio station.

The organization's top leader, Khaled Mashaal, called on Palestinians to renew their battle against Israel.

"This is the time for a third uprising," he said in Damascus, Syria.

In Gaza, a 57-year-old shopkeeper wailed for his lost son as he sat in the middle of the street not far from a bombed-out security compound.

"My son is gone, my son is gone," cried Said Masri, who said he had sent his nine-year-old out to purchase cigarettes minutes before the airstrikes began and could not find him.

"May I burn like the cigarettes, may Israel burn," Masri moaned.

While Egypt opened its Rafah border to allow ambulances to transport the wounded to Egyptian medical facilities, the number of wounded reportedly outnumbered available ambulances.

"There are heads without bodies…There's blood in the corridors. People are weeping, women are crying, doctors are shouting," said Ahmed Abdel Salaam, a nurse at Gaza's main treatment centre, Shifa Hospital.

'A time for fighting,' Barak says

Israeli Defence Minister Ehud Barak said on Saturday that Israel's air offensive against militant sites in Gaza "will widen as necessary."

"There is a time for calm and there is a time for fighting, and now is the time for fighting," he told a news conference in Tel Aviv.

Israeli Foreign Minister Tzipi Livni warned that the next targets could be Hamas's political leaders.

"Hamas is a terrorist organization and nobody is immune," she said.

Thousands of Gazans received Arabic-language cellphone messages from the Israeli military late Saturday, urging them to leave homes where militants might have stashed weapons.

Israel also told its civilians near Gaza to take cover as Palestinians began retaliating. One missile hit the town of Netivot, killing an Israeli man and wounding six people, rescue services said.

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Israeli hospital patients and their families wait in a bomb shelter in Ashkelon Barzily hospital, in southern Israel, on Saturday. ((Tsafrir Abayov/Associated Press))

Defence officials have estimated that Hamas is capable of firing dozens of rockets a day.

Earlier in the week, Olmert urged Gazans to stop Hamas fighters from firing rockets and mortars into Israel. Defence officials had warned of an impending offensive, saying an Israeli operation would likely begin with surgical airstrikes against rocket launchers and continue with a land invasion.

Gaza and the surrounding area have seen a steady escalation of violence since a six-month ceasefire between Israel and Gaza militants ended last Friday. Since then, Gaza militants have fired about 300 rockets and mortars at Israeli targets, the Israeli army said.

While the ceasefire brought some calm to the coastal area around Gaza and saw fewer rocket attacks on southern Israeli towns, both Israel and Hamas have accused one another of violating the terms of the agreement.

Hamas complained that Israel never fully opened its border crossings with Gaza to permit flows of international aid and fuel into the impoverished territory, which sparked shortages of goods and widespread blackouts.

Israel, which left Gaza in 2005 after a 38-year occupation but still controls its border crossings, says Hamas used the truce to replenish its arsenal with arms smuggled in through dozens of tunnels under the territory's sealed border with Egypt.

EU officials urges ceasefire

Saturday's attacks quickly drew calls for restraint from across the world.

European Union foreign policy chief Javier Solana has called for an immediate ceasefire.

"We are very concerned at the events in Gaza. We call for an immediate ceasefire and urge everybody to exert maximum restraint," his spokesman said.

The United States urged Israel to avoid civilian casualties and said Hamas must stop rocket attacks into Israel for the violence to cease.

"Hamas' continued rocket attacks into Israel must cease if the violence is to stop. Hamas must end its terrorist activities if it wishes to play a role in the future of the Palestinian people," White House spokesman Gordon Johndroe said.

Speaking in Cairo, Arab League chief Amr Moussa condemned the violence and said he also had called for an emergency meeting of the Arab Foreign Ministers Council on Sunday.

Canada's foreign affairs minister, Lawrence Cannon, also issued a statement in which he expressed concern over the loss of life and urged renewed efforts to reach a ceasefire.

"Israel has a clear right to defend itself against the continued rocket attacks by Palestinian militant groups which have deliberately targeted civilians.

"First and foremost, those rocket attacks must stop. At the same time, we urge both sides to use all efforts to avoid civilian casualties and to create the conditions to allow safe and unhindered humanitarian access to those in need in Gaza."

With files from the Associated Press