The rising civilian death toll in Israel's campaign in Gaza highlights the pitfalls of Israel's powerful army using lethal force against often invisible Hamas guerrillas taking cover among civilians.

On Tuesday, only two of 71 people killed were confirmed militants and the United Nations called for an investigation into the growing civilian casualties.

An Israeli defense official confirmed that the military has adopted tougher tactics to prevent the killing or capture of soldiers. One concern is that troops could be lured toward booby-trapped houses, said the official, who spoke on condition of anonymity because he was not authorized to discuss military strategy.

However, the images of maimed or bloodied Palestinian civilians, including children, is likely heighten international pressure on Israel to abort the offensive before it has obtained its main objective — hitting Hamas hard so it will halt rocket fire on Israeli border towns.

In Israel's campaign against Lebanese Hezbollah guerrillas in 1996, errant Israeli artillery shells killed 91 Lebanese civilians at a U.N. base near the village of Qana, turning initial international support for the operation into harsh criticism. In 2006, Israeli shells killed 18 Palestinians in the Gaza town of Beit Hanoun.

In the single deadliest strike of the current Gaza offensive, at least 30 Palestinians were killed and 55 wounded Tuesday when Israeli mortar shells hit near a U.N. school where hundreds of civilians had sought refuge from the fighting. Israel said Hamas militants had fired mortar shells from outside the school, drawing return fire.

Palestinian residents speaking on condition of anonymity because of fear of retribution said that several militants ran toward the crowd, trying to use it as cover, when the first Israeli mortar shell missed them.

Israel said Hamas militants were among the casualties, but many of the dead appeared to be civilians, including boys and teenagers. In hospital footage shown on Hamas' Al Aqsa TV, frantic medics unloaded men who had been stacked in the back of an ambulance, three high, and were dragged without stretchers.

Gaza militants used to wear black or khaki uniforms, but since the start of the Israeli offensive have been operating in civilian clothing, blending into crowds.

Israeli military officials complain that Hamas is hiding behind civilians. "The civilians are unfortunately being hurt, not because we are targeting them but because of the Hamas policy to have all of its facilities in densely populated areas," said an Israeli military spokesman, Brig. Gen. Ilan Tal.

Ostensibly, Hamas' ragtag force of about 20,000 fighters, armed mainly with assault rifles, mortar shells and rocket-propelled grenades, is no match for Israel's high-tech army.

If the two were facing each other on open ground, "it would be over in two minutes," said Bob Ayers, a London-based security analyst and former U.S. intelligence official.

However, with Israeli forces moving deeper into Hamas' home turf after a week of aerial assaults, the Islamic militants are improving their odds. Hamas has boasted that it prepared deadly surprises for Israeli forces and Tal said that includes booby-traps and tunnels.

Israel has more than two decades of experience fighting Arab guerrillas in urban settings, particularly in Lebanon and to a lesser extent in the Palestinian territories. Israel has learned from previous rounds in Gaza and the 2006 war against Hezbollah, said Shlomo Brom, a former senior Israeli military official.

The underbelly of Israel's Merkava tank has been strengthened, he said, after Gaza militants succeeded several times in recent years to detonate bombs under tanks, destroying them.

In 2006, Hezbollah surprised Israeli forces by firing large numbers of anti-tank missiles with deadly results, but Hamas is believed to have far fewer, said Brom and other experts. Hamas also has heavy machine guns and has tried, but so far failed, to shoot down Israeli attack helicopters.

Despite its clear military superiority, Israel has been wary to launch the ground offensive, for fear of getting bogged down in the alleys of Gaza, with many casualties among Israeli forces. The order to move in came only after a week of massive air strikes failed to halt Hamas rocket fire on towns in southern Israel.

For now, Israeli tanks are still several hundred yards away from densely populated areas, targeting militants from a distance. Using optical sightings, tank shells are generally accurate, but are also very lethal or could hit the wrong target, said Ayers.

On Monday, three Israeli soldiers were killed by friendly fire when a tank crew took aim at the wrong house. In a separate incident, another soldier was killed by an errant tank shell.

"With tanks in an urban environment, the opportunity for miscalculation is magnified significantly," said David Hartwell, Middle East and North Africa editor for Jane's Country Risk.

Since the start of the ground offensive Saturday, more than 120 Palestinian civilians have been killed — out of a total of more than 600 deaths overall in 11 days of fighting.

Most of the civilians killed in recent days were hit by tank and artillery fire, according to Palestinian medics. The dead include parents and five children killed when a tank shell hit their home, and three men struck by a shell on their way to a condolence visit.

The U.N. has called for an investigation of the rising toll among civilians, after several of its installations were hit. U.N. officials noted that Israel's military had been given the exact locations of U.N. buildings.

Israel insists its doing its best to minimize civilian casualties. However, Chris Gunness, a spokesman for Gaza's main U.N. aid agency, said "it would have to be known to any military planner as a matter of certainty that such and overwhelming and disproportionate use of force would inevitably lead to civilian casualties."