Israel has opened a new phase in its war on Hamas by expanding ground operations inside Gaza, but analysts warn the campaign is its riskiest in half a century with fallout threatening the whole Middle East.

Thousands of civilians have already died in Gaza and Israel since Hamas launched its shock 7 October attacks. And the United Nations has led warnings that thousands more will die as Israel sends troops and tanks farther into Gaza.

That adds to Western fears that Iran-backed Hezbollah could open a new front on the Lebanese border. Officials say Israel does not want to stay in Gaza and there are also concerns over who will administer the territory and pay for its reconstruction after the guns fall silent.

With Gaza's hospitals and food supplies devastated Israel's Arab neighbors worry that the images of Palestinian suffering could trigger a pro-Hamas backlash in their own countries.

Israel's Prime Minister Benjamin Netanyahu vowed an "iron fist" treatment of Hamas after its fighters attacked communities across southern Israel. They killed 1,400 men, women, and children, and took back at least 230 hostages, according to Israel.

Hamas authorities say more than 8,000 people, half of whom are children, have died in thousands of Israeli air strikes since the start of the war.

Images of the devastation have fuelled anger in many countries. Now tens of thousands of Israeli soldiers are waiting on the border for the toughest stage of a war that Netanyahu warned would be long and difficult.

"We are going to see a lot of carnage, we are going to see a lot of horrible things," said Edward Djerejian, a former US assistant secretary of state and ambassador to Israel, who bemoaned the lack of a political initiative to end the crisis.

This showdown is Israel's most perilous since the 1973 Arab-Israel war, when it was also taken by surprise, according to Jonathan Rynhold, a specialist on the Israel-Palestinian conflict at Bar-Ilan University near Tel Aviv.

He said Israel will have to be ready for major casualties which will be worse if Hezbollah turns its near-daily artillery exchanges with Israel into all-out conflict.

Israeli troops have been given special training for urban warfare in the Palestinian territory's narrow rubble-strewn streets and a huge network of Hamas underground tunnels that they call the "Gaza metro".

"If Israel follows through on the stated aim of destroying Hamas military capabilities in the Gaza Strip and overthrowing its regime, then the scale and length of this war will be much bigger and much longer" than the four previous Gaza wars since 2005, the longest of which lasted seven weeks, said Rynhold.

The expansion of ground operations "will be the critical moment as to whether a second front opens with Hezbollah and that is a higher risk" than in previous wars, he added.

Open conflict with Hezbollah could drag in the United States and would mean Israel having to accept "a scale of destruction that it has never experienced before," Rynhold said.

The United States and the European powers have given Netanyahu strong support while urging him to limit the civilian casualties that fuel Arab anger and open up Gaza to more humanitarian aid.

US President Joe Biden has sent two aircraft carrier groups to the eastern Mediterranean and warned Hezbollah and others to stay out. But he has also urged Israel to curtail its "rage".

H.A. Hellyer, a security specialist for the Royal United Services Institute in London, said that Israel is not doing enough to head off new war fronts.

"There is a risk of extension of the conflict," he said. "Israel is prioritizing revenge and retaliation over all else, as far as we can see from the statements of senior Israeli officials."

Israel has vowed to destroy Hamas but while it may take out the Islamist group's leadership, analysts warn it is unlikely to take away its support base.  

Khalil Shikaki, director of the Palestinian Center for Policy and Survey Research in Ramallah, said Hamas is "probably a lot more popular today" in the Arab region than before the attacks.

Hamas will retain significant influence in Gaza after the war, he predicted.

Israel has not had the public sympathy it expected in Western countries. And with Iran and other rivals watching closely, it must now prove that it remains the Middle East's undisputed military power.

Israel's reputation "depends on its projection of strength, its swagger," said Laura Blumenfeld, a former US State Department adviser on Israel-Palestinian negotiations and now a security specialist at Johns Hopkins University.

If the war tarnishes "its sheen of deterrence" it will look "weak" before its rivals and countries that might be considering normalizing ties, she added.

Rynhold said: "It is not that Israel will lose but that the price of victory will be very high."