When Israeli battle tanks rumbled up to this Palestinian refugee camp, they were met with bursts of gunfire, a road strewn with homemade land mines, intersections blocked by burning barricades — all in all, exactly the reception that would have been expected.

Israeli soldiers have at times subdued entire Palestinian cities with only minimal resistance, but refugee camps are another story. Crowded, poor and simmering with fury, the camps have proven the deadliest battlegrounds of Israel's 12-day military offensive in the West Bank.

That was the case again Tuesday, when Palestinian militants ambushed Israeli soldiers during intense fighting in the Jenin refugee camp, firing from rooftops and setting off explosions that collapsed a building on troops in an alley. Thirteen Israeli soldiers were killed and nine wounded — the single bloodiest attack against Israeli troops since fighting between the two sides erupted 18 months ago.

Palestinians, too, are suffering some of their heaviest casualties in the camps. By the estimates of both sides, more than 100 Palestinians — some gunmen, some civilians — have been killed in the Jenin camp during the past week.

Other refugee camps have been dealt more glancing blows, like the Balata camp in the West Bank's largest city of Nablus, where residents say tank shells shattered 25 homes and killed six people, or the Al-Ain camp, also in Nablus, where gunbattles broke out twice in the past week.

Israeli military incursions into refugee camps are a relatively new phenomenon in the current conflict, taking place only in the last six weeks. But in many ways, camp warfare is the inevitable culmination of Israeli Prime Minister Ariel Sharon's vow to pursue Palestinian militants wherever they try to seek sanctuary.

Most of the refugee camps are strongholds of radical Palestinian groups like Hamas, Islamic Jihad and, more recently, the Al Aqsa Martyrs' Brigades, which have carried out dozens of attacks against Israelis.

But first and foremost, the refugee camps are home to Palestinian civilians — more than 600,000 live in 27 refugee camps in the West Bank and Gaza Strip, according to the U.N. agency for Palestinian refugees. Far from being the temporary tent communities the name might suggest, they are cities within cities, full-scale urban zones with schools and hospitals, clinics and mosques.

Without exception, the camps are extremely crowded. Families of 10 or 12 children are not unusual. Most of the camps stayed within their original boundaries as their populations have swelled over the years, with families adding rooms to their cinderblock shanties if they can, until only the narrowest of alleyways remain between rows of houses.

The sheer density of humanity is what makes the camps so dangerous during fighting — primarily for civilians, but also for well-equipped, well-protected Israeli soldiers. The troops can't drive tanks and armored personnel carriers through camp alleyways, so if they want to search house-to-house for gunmen and weapons, they must fan out on foot.

The soldiers' typical method of making their way through the camps is to burst into one home and batter holes through the wall to the next. This shields them from ambush in the open, but is terrifying and sometimes fatal for families whose homes are thus invaded. Last month, in the Deheishe refugee camp in Bethlehem, a young mother bled to death in front of her horrified family after she was hit by shrapnel when soldiers used an explosive charge to blow open the door of their home.

"Every night in my nightmares I imagine that this is happening — that they are breaking open the door to my house," said Hussein Hosain, who lives in Nablus' Askar camp, also the scene of clashes this week.

If gunmen holed up in a camp are offering fierce resistance — as they did in Jenin and elsewhere — the army routinely brings withering firepower to bear. At such close quarters, battlefield weapons like tank shells, heavy machine gun fire and helicopter-fired rockets — even if aimed with some precision at militants who are shooting at soldiers — invariably kill and maim many civilians as well.

Even when Israel batters a refugee camp into temporary submission, vanquished fighters can exact their revenge. There have been several close-range suicide attacks by Palestinians in the Jenin camp, Israeli commanders say, including a woman who blew herself up with an explosives belt when troops came to her door.

The camps, which are home to the descendants of Palestinian refugees who fled or were driven from their homes during Israel's war of independence 54 years ago, were a driving force behind the last uprising, or intefadeh, against Israel, from 1987 to 1993.

That conflict broke out in the Jebaliya refugee camp in Gaza, the biggest in the Palestinian territories. Then, as now, men and boys from the refugee camps — bitter, dispossessed, believing they had little to lose — were disproportionately represented in the ranks of stone-throwers and fighters.

Israel claims the well-fortified backstreets of refugee camps are prime locations for bombmaking factories — which has sometimes been backed up with evidence, sometimes not.

During a deadly assault in Jebaliya last month, Israeli rocket and missile attacks destroyed several factories Israel said were making munitions. One of the wrecked industrial sites, Palestinian neighbors maintain, was a ceramics factory and another retooled old car engines.

Because those living in the camps tend to be among the poorest of the Palestinian poor, the property destruction that accompanies a military incursion is a devastating blow to families already struggling to survive. Even those who escape an attack unharmed can be seen weeping and cursing over the loss of meager possessions.

"I live in fear that this will happen to us — my brother and I worked all our lives to build this house," said Ghassan Osman, 26, from the Balata camp, who said tank shells had landed close by several times in the past few days.

"It is such a poor house, but it is all we have," he said, adding that the two brothers, together with their wives and children, had only recently been able to move from a tin-roofed shanty elsewhere in the camp to a cement-block house with a proper roof. "If we lose it, I will feel as if we have lost our lives."