It was always lethal territory. And yesterday the Israeli army endured the worst day in its recent history

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The Palestinian refugee camps of the West Bank have always been lethal territory for Israel's forces. The dark, winding alleys and tightly packed buildings offer perfect cover for armed men. Troops, entering on foot because the roads are too narrow for armoured vehicles, are at constant risk of being shot.

These places represent the boiling core of opposition to Israel. Refugee children, reared in squalor, learn chants of nationalism and war almost as soon as they can talk. Portraits of the assassinated leaders of Hamas adorn the walls. Graffiti shows exploding Israeli buses, and heroic, blood-soaked "martyrs", suicide bombers who have atomised themselves in the name of destroying their enemies, regardless of the fact that they are civilians.

And yet – until yesterday's devastating booby-trap killed 13 of their reserve soldiers – Israel's commanders will have been congratulating themselves. They had entered the lion's den, fighting house-to-house in Nablus's Balata camp and in Jenin in some of the most ferocious battles of the 18-month conflict. They have killed many scores of Palestinians – some gunmen, some security forces defending their turf, but some civilians. They have also inflicted physical injury on many hundreds, and misery on thousands of people, mowing down swaths of homes, cutting them off from medical supplies and, in some accounts, using them as human shields.

Yet the army's own losses were surprisingly low. It seemed pleased with its performance.Ordinary Israelis, whose views hardened dramatically after the sickening Passover suicide bombing, which killed 27 – had begun to remark on the success of the offensive, the so-called Operation Defensive Wall. News reports of Palestinian corpses piling up in the streets brought only subdued public comment. After seeing his position in the polls dip, Ariel Sharon's position was strengthening, emboldening him to draft in several virulent right-wingers to his cabinet. The picture has now changed. Until yesterday, the Israeli Defence Forces had lost only 13 men in 11 days, after deploying some 30,000 troops in the West Bank, with some 2,000 tanks. "In pure military terms, it was an efficiently run operation," said a Western diplomatic source. "Losing so few, when you have so many troops and house-to-house fighting, is an achievement, no matter how wrong-headed it is as a strategy."

But yesterday 13 soldiers were killed in an elaborate ambush set by Palestinian fighters in a camp in Jenin – the deadliest attack of the intifada, and one of the worst days in the army's recent history.

When the Israeli Defence Forces first stormed into the camps last week, backed by Cobra helicopters and tanks, they found the streets littered with booby-traps, from gas canisters primed to explode, to home-made mines, but they were largely ineffective.

Yesterday's barrage of bombs was different. Reports said some of the soldiers were killed when a booby-trap detonated in a building, and others died when gunmen opened fire on them after they went to rescue their comrades.

The attack included several bombs, including, the Israeli army said, suicide bombers. All the dead were reservists from the Infantry Brigades. They had been searching the yard of a house in the refugee camp.

At first, the Israelis feared several soldiers had been taken hostage, but this seems to have been unfounded. There were Palestinian reports that the Israelis appealed for a ceasefire to allow them to recover the dead and wounded. Seven soldiers were injured, one critically.

It was not the first serious blow the Israeli army has suffered during the current conflict. In February, the Palestinians managed for the first time to destroy a Merkava tank, using a home-made mine made from a water tank – a feat that they repeated a few weeks later, destroying the vehicle's reputation for invincibility. There was a public outcry among Israelis when seven soldiers were killed on 3 March by a single sniper using a Second World War rifle, firing from a hilltop at an exposed checkpoint in the West Bank.

Although Israels regular army units are regarded as competent, doubts abound about the commitment of its reservists. Some 400 have signed a petition saying they will not serve in the occupied territories, arguing that the occupation is corrupting the country.

Before the Passover bombing, the loss of 13 soldiers would have prompted more opposition to Mr Sharon's strategy, in which he appears to be trying to use military force to consolidate Israel's hold over most of the West Bank, systematically destroy the institutions of Palestinian statehood, and confine the Arabs of the West Bank into autonomous urban pockets.

But the signs last night were that the soldiers' deaths would not alter the hardline mood in Israel. Mr Sharon cited them as a reason to carry on with the offensive, ignoring a wave of international pressure.

Jack Straw, Britain's Foreign Secretary, added his voice yesterday to US calls for an immediate withdrawal. He described Israel's response – a token pull-out from two towns while entering a third – as "not good enough".

The Jenin ambush has come as a shattering blow to Israeli morale. That much was obvious from the conduct of the soldiers who stopped us getting into Jenin to find out what had happened. We were ordered out of our cars – hands aloft – while a tank pointed its barrels at us. "Sorry," said one soldier, "but this is not a good day."