Hamas may be militarily outnumbered and outgunned by Israel, but it has the stamina and training to turn the Gaza ground invasion into a protracted urban war, experts warned Tuesday.

Six days after Benjamin Netanyahu ordered ground forces across the border into Gaza, the death toll has climbed to more than 600 Palestinians and 29 Israeli soldiers - more than twice as many military casualties as in the last Gaza ground war of 2009.

Hopes that the invasion by Israeli Defense Forces would swiftly end the tit-for-tat aerial exchange of Palestinian rockets and Israeli missiles look to have been dashed as gaps between the sides remain wide.

“Hamas’ strategy seems to be to suck Israeli forces into a situation where their numerical and qualitative advantages are neutralized, by tunneling into areas where they can attack or laying various traps,” said Chris Doyle, director of the Council for Arab-British Understanding. “It is trying to conduct a classic urban guerrilla conflict for which it has no doubt been preparing for some time.”

The Israeli prime minister's stated aim is to smash the tunnels that allow militant Palestinians to cross the border and attack Israeli forces and to restore “sustainable quiet” for citizens in southern Israel living under the threat of rocket fire. "It is not possible to deal with tunnels only from the air. It needs to be done also from the ground," the Israeli PM said Friday.

"There are booby-trapped houses everywhere and the Hamas fighters are incredibly entrenched"

But even as it suffers heavy losses, Hamas’ top leader in Gaza reiterated his goal of forcing Israel and Egypt to lift the long-standing border blockade. "We cannot go back, we cannot go back to the silent death" of the blockade, Ismail Haniyeh said. "Gaza has decided to end the blockade by its blood and by its courage."

“Hamas has not even been pinpricked by the number of its forces killed inside Gaza,” said Daniel Nisman, president of The Levantine Group, a Tel Aviv-based risk analysis consultancy. “The stamina of Hamas is probably greater than Israel’s.”

Israel vs. Hamas in numbers

Israel has 176,500 active military service personnel comprising 133,000 soldiers, 9,500 in the navy and 34,000 in the air force, according to Jane’s The Military Balance 2014. Among them are eligible volunteers from around the world, including the United States. It also has reserve forces totalling 465,000, most of which are soldiers.

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Even taking into account recent budget cuts, the IDF "remain the most capable force in the region [Middle East], with the motivation, equipment and training to considerably overmatch the conventional capability of other regional armed forces,” according to Jane’s.

The military strength of Hamas is harder to quantify. “Precise personnel strength figures for the various Palestinian groups are not known,” says Jane’s, which lists Palestinian forces under the heading “paramilitary” and puts the figure at 56,000. However, most of those are under the management of the Palestinian Authority which controls the West Bank.

In Gaza, Hamas has a direct fighting capability estimated at about 20,000 personnel, including members of its military wing, the al-Qassam Brigades. “That's 10,000 full-time soldiers and another 10,000 policemen who have wartime mobilization orders,” Nisman said. Outside of that are other militant groups including Palestinian Islamic Jihad, which has several thousand members, and the Popular Resistance Committees.

The lines between militants, soldiers and police – and even civilians - are blurred, Nisman said. “Hamas is basically an army,” he said. “Some security forces moonlight as al-Qassam fighters.”

Hamas is “organized in brigades, platoons and companies just like a military,” said Brigadier General Asaf Agmon, head of the Fisher Institute for Air and Space Strategic Studies, which was established by Israel’s Air Force Association. Having small units that can operate autonomously, carrying out their own individual attacks, is one of the reason Hamas is regarded by Israel as a terrorist organization. Agmon said some Palestinian militants who crawled out of a tunnel on Monday night had disguised themselves by wearing IDF uniforms.

The conflict on the ground

About four of five IDF infantry divisions are thought to be inside Gaza. While IDF forces might be better-equipped, the ground battle is proving tough – and very deadly. Both sides show evidence of miscalculation.

In the Shejaia neighborhood, east of Gaza City, heavy Israeli airstrikes and shelling has led to the deaths of well over 100 Palestinians since Sunday. IDF troops in that area are also encountering ambushes and anti-tank missiles, including an improvised explosive device that killed 13 soldiers in a vehicle over the weekend.

Q&A: The History Behind Israel’s Gaza Offensive

In addition, Hamas has made almost nightly attempts to infiltrate Israel by crawling through underground tunnels.

“It is probably the most intense combat zone anywhere in the Middle East, more intense than many places in Syria or Iraq,” said Nisman. “There are booby-trapped houses everywhere and the Hamas fighters are incredibly entrenched. There is a real question about whether Hamas will retreat into Gaza or not. This is a more aggressive Hamas than we’ve seen and a more aggressive Hamas than Israel anticipated.

“Hamas has been planning this conflict for months. There were testing long-range rockets in the Sinai and building these tunnels to kidnap [Israeli] soldiers - maybe they didn’t think the IDF would invade in the way that it has done.”

The scope of IDF activity has also widened as the conflict stretches on. “They went past their ‘A list’ of target sites a long time ago,” said Doyle. “They may be tempted into taking on sites that have less and less legitimacy.”

What next?

Secretary of State John Kerry was in Cairo Tuesday, sent by President Barack Obama to seek a cease-fire between Israel and Hamas. But the prospects of a long-lasting deal appear thin.

A senior Israeli official told NBC News on Tuesday that there was “no lasting cease-fire on the horizon” and no sign of Israel’s military mission being completed. “Hamas is not ready to agree on terms that are anywhere near what we can accept,” the official said.

“Nobody wants to agree to a cease-fire,” Nisman said, adding that the worst outcome for Netanyahu would be a deal that imposed the same conditions that ended the last round of fighting in 2012. “That would be political suicide,” he said, noting that most Israelis favor an outcome that conclusively prevents another cross-border conflict.

“Both sides need to be able to declare some kind of political victory but it is not going to resolve the underlying issues,” said Doyle. “There is always going to be the threat of kidnappings or other forms of action by Hamas and Israel will always come over the border when it sees fit and in all of this civilians are the big losers.”

Agmon said Israel was “determined” to deal with the tunnels that pose a threat to Israelis living in border areas, if necessary expanding the ground invasion by “a few more days or even weeks.” He said: “Even if there will be cease-fire I am sure we will insist, ‘no more tunnels’." Otherwise, he said, “it is like going to sleep and not knowing who will pop up in your bedroom.”

Ben Plesser and Polly DeFrank of NBC News contributed to this report.