JERUSALEM -- A network of tunnels Palestinian militants have dug from Gaza to Israel -- dubbed "lower Gaza" by the Israeli military -- is taking centre stage in the latest war between Hamas and Israel.

Gaza's Hamas rulers view them as a military game changer in its conflict with Israel. The Israeli military says the tunnels pose a serious threat and that destroying the sophisticated underground network is a key objective of its invasion of Gaza.

Israel has known about the tunnels for several years, but has been hard-pressed to find an effective way to block them. Now it is counting on its ground war to at least reduce the threat.

"Israel knew there was a problem with the tunnels, but it didn't internalize their significance," said Shlomo Brom, a retired Israeli general. "At any given moment, Hamas could send dozens of militants through separate tunnels to attack communities in Israel."

Gaza has two sets of tunnels -- those reaching Egypt and those reaching Israel.

The underground passages to Egypt are meant to bypass a border blockade on Gaza that was tightened by Israel and Egypt after Hamas seized the territory in 2007. The tunnels provide an economic lifeline and are used to deliver building supplies, fuel, consumer goods and even cattle and cars.

In some of those tunnels, Gaza militants received weapons and cash from their patrons abroad, particularly Iran. Egypt has destroyed virtually all of the tunnels over the past year, driving Hamas -- which was taxing the smuggled imports -- into a severe financial crisis.

Separately, Hamas significantly expanded its network of tunnels from Gaza to Israel after a major Israeli ground offensive that ended in January 2009.

Several senior Hamas officials have told The Associated Press that the expansion of those tunnels was part of a broader shift in military tactics after 2009, with input from the Hezbollah guerrilla group in Lebanon, which has fought Israel repeatedly.

"There are thousands of resistance fighters working underground and thousands others working above the ground, to prepare for the upcoming battle," Ismail Haniyeh, a top Hamas leader in Gaza, said earlier this year. "No one can imagine what the resistance is ready to do to confront the occupiers."

In addition to the tunnels, Hamas also boosted its arsenal of anti-tank rockets, which had proven effective in Hezbollah's battle with Israeli ground troops in Lebanon in 2006, said the officials, who spoke on condition of anonymity because they were not permitted to discuss military strategy with reporters.

Hamas also moved many of its rocket launching sites and storage sites underground, making it more difficult for Israel to target them. Since the current round of Israel-Hamas fighting began on July 8, Gaza militants have fired more than 2,000 rockets at Israel and repeatedly tried to sneak into Israel through tunnels.

Such attempts have stoked new calls in Israel to destroy the tunnels, something that can only be done by ground forces. This helped rally widespread public support for the Israeli invasion of Gaza, despite the rising number of Palestinian deaths.

More than 800 Palestinians have been killed, three-fourths of them civilians, and most since Israel's ground operation aimed at hitting the tunnels began eight days ago, according to U.N. figures. Thirty-four Israelis, including 32 soldiers, and a Thai worker have been killed.

Israel says Hamas has dug dozens of tunnels, linking them to each other as well as to rocket manufacturing sites, maintenance facilities, launch sites and command and control centres. It says the tunnels are meant to facilitate mass attacks on Israelis as well as kidnappings, a tactic that Hamas has used in the past.

In 2006, Palestinian militants burrowed under the border to an Israeli army base, killed two soldiers and captured a third, Sgt. Gilad Schalit. After being held captive in Gaza for more than five years, he was exchanged for more than 1,000 Palestinian prisoners in 2011.

Soldiers have uncovered 31 tunnels in the current round of fighting, the military said Thursday.

Palestinian militants trying to sneak into Israel through the tunnels have been found with tranquilizers and handcuffs, an indication that they "intended to abduct Israelis," according to the military.

"Hamas has dug terrorist tunnels under hospitals, mosques, schools, homes, to penetrate our territory, to kidnap and kill Israelis. Now, in the face of such wanton terrorism, no country could sit idly by," Prime Minister Benjamin Netanyahu told visiting U.N. Secretary-General Ban Ki-moon this week.

Israelis countrywide have been granted a front-row seat to the rising threat thanks to footage released by the Israeli military, which has shown heavily armed militants, toting rocket-propelled grenades and automatic weapons, lying low in the weeds as they are spotted by army lookouts.

In some cases they have been forced back into the tunnels and in others they have been killed in shootouts with Israeli forces or airstrikes. While a number of troops have been killed in the altercations, militants have not reached nearby communities.

The visuals have spooked the country and fueled public support for the latest incursion, which is backed by a majority of Israelis.

Still, some Israelis have questioned how Israel, which has minimized casualties and damage from Hamas' rocket threat with its Iron Dome missile defence battery, has been unable to stanch the infiltrations. Beyond the casualties, the incidents have led widespread road closures and forced thousands of residents to lock themselves inside for safety.

Brom, an analyst at the Institute for National Security Studies think-tank , said he expected a greater push in Israel following the war to find a comprehensive remedy to the tunnel threat.

Atai Shelach, a former commander of the military's combat engineering unit, Yahalom, said ground forces could inflict some damage and deter Hamas, but that more is needed for the longer term.

Shelach said Israeli companies are working to find a defence against the tunnels, but have not yet succeeded in developing an adequate solution.

Ideas that have been floated in the past include digging a canal filled with sewage along the border meant to thwart infiltrations, but the task would be expensive and may not suit the sandy soil around Gaza. A variety of technologies could help detect tunnel activity through acoustic or seismographic readings, but none are foolproof.

"We need a game changer like Iron Dome," Shelach said. "It's not a question of money or resources. It just needs to be made a national priority."