Israel has thousands of troops massed for a ground offensive on Gaza that would aim to deal a hammer blow to Hamas and re-establish Israel's military credentials with its other foes, experts said.

After Israel's 2006 war in Lebanon, widely condemned as a fiasco, this campaign has been scrupulously prepared and few experts believe Hamas can halt the waiting army.

The number of troops and tanks along the 60 kilometre (37 mile) border is a military secret but Israeli leaders say the force is ready and local media say the assault is imminent.

Israel launched air strikes and a naval bombardment one week ago in response to weeks of militant rocket fire from Hamas-run Gaza.

Prime Minister Ehud Olmert has called it "iron fist" treatment.

Israeli missiles have killed more than 420 Palestinians -- including top Hamas leader Nizar Rayan -- while about 300 militant rockets have killed four people and wounded dozens in Israel.

Some analysts say Israel expects dozens of its soldiers to be killed in any ground offensive, but the government insists it wants to stop the rockets.

"Hamas must understand that if it launches rockets in the future it will have to pay a prohibitive price," said Shabtai Shavit, a former head of the Mossad foreign intelligence service and now an advisor to the Israeli National Security Council.

"Israel does not intend to reconquer the Gaza Strip," Shavit told AFP. Any ground campaign "would mean going in and inflicting additional damage and cost to Hamas."

Asked whether this means killing more Hamas leaders or just destroying infrastructure, he said: "Everything".

He predicted a war could last weeks with the ground offensive quickly followed by a phased withdrawal if a "political agreement" with Hamas can be concluded.

Uzi Dayan, a former deputy chief of Israel's general staff and national security advisor to prime ministers Ehud Barak and Ariel Sharon, said the army must control areas of Gaza from which rockets are being launched and "put more pressure on the Hamas leadership and Hamas activists."

He said the Israeli army would need to take control of key areas in Gaza, right up to the Egyptian border where a system of tunnels are used to smuggle weapons and other supplies into the territory.

Dayan told AFP that detaining Hamas leaders and activists will encourage ceasefire negotiations. "You need some cards, it is not enough to just say 'please stop the bombing'."

Experts also said Israel wants to send a message to Iran, the Hezbollah militia in Lebanon and other enemies that after the disastrous 2006 war in Lebanon, the Middle East's most powerful army is to be feared again.

The main lesson, according to Shavit, Dayan and several Israeli officers, is that an invading force cannot rely on the air force alone to beat an enemy. Israeli commanders were strongly criticised for waiting too long to send in ground troops in 2006.

"Our military has learned the lessons from the Lebanon campaign," said Shavit. "We need this campaign to re-establish the deterrent" after the failures of Lebanon.

Efraim Inbar, director of Bar-Ilan University's Begin-Sadat Centre for Strategic Studies, agreed. "The Israeli army needs to address the problem created to its deterrence in 2006," he said.

"The army has to change its image among its enemies that it is afraid to use ground forces.

"There is no choice but to use ground forces. It does not necessarily need to be a large scale invasion, maybe just special units or localised incursions but the army needs boots on the ground."

Israel will also be under pressure to make the campaign as short as possible, the experts said. Prime Minister Olmert said this week that Israel was "not interested in conducting a long war."

"Israel is much stronger than Hamas. We can continue the operation longer than Hamas can, but on the other hand we do not have all the time in the world," said Dayan, the former national security advisor.

He said international pressure will increase for a ceasefire and that domestic pressure will grow while hundreds of thousands of Israelis are in the line of fire of Hamas rockets.