Anas Mansour sleeps in his street clothes in a south Gaza refugee camp, with his ID in his pocket so he can flee quickly if fighting gets worse. In Gaza City, the 10 members of the Karam family huddle in their hallway at night, kept awake by artillery fire booming outside.

Israeli's offensive against Gaza's Hamas rulers has moved from pinpointed air strikes to artillery shelling and ground fighting, though, for now, largely in open areas.

Still, Gaza's civilians are increasingly exposed to the violence. At least 40 civilians, including three medics, have been killed since Israel launched its ground invasion on Saturday evening, Gaza health officials said.

Aerial bombardment

Israel says eight days of aerial bombardment, followed by the ground invasion, are aimed at undermining Hamas' ability to fire rockets at the Jewish state.

More than 500 Palestinians have been killed since the operation began, at least 100 of them civilians, say Palestinian and United Nations officials. In the same period, three Israeli civilians and two soldiers have been killed.

In Gaza, residents said the offensive's first stage came through its sounds: the buzz of warplanes, the boom of tank shells clearing the way for thousands of advancing soldiers and the wail of ambulance sirens.

Israel's guiding principle is to move in with full force and try to minimise Israeli casualties, Israeli military correspondent Alex Fishman wrote in the daily Yediot Ahronoth.

"We'll pay the international price later for the collateral damage and the anticipated civilian casualties," Fishman wrote.

Israel says there is no humanitarian crisis in Gaza, noting that it has continued to allow humanitarian supplies into Gaza throughout the crisis. It says its airstrikes target only Hamas installations and leaders, but bombs have also destroyed or damaged adjacent houses.

Lubna Karam, of Gaza City, said air strikes had shattered her home's living room windows days before, letting cold air pour in. She said she feels under threat at all times, and her family has trouble sleeping.

"We keep hearing the sounds of aeroplanes and we don't know if we'll live until tomorrow or not," said Karam, 28.

Mansour, 21, of the Rafah refugee camp on the Gaza-Egypt border, described watching his neighbour pile a mattress and blankets on a donkey cart to flee, but hadn't decided if he'd do the same. "Where can we go? It's all the same," Mansour said.

The current Israeli operation came at the end of an ever-tightening blockade of the seaside territory, imposed after the violent Hamas takeover of Gaza in June 2007. The borders were virtually sealed in the last two months, leading to shortages of cooking gas and basic foodstuffs.

Sewerage system collapses

The Israeli human-rights group Gisha said Israeli air strikes have left Gaza's water and sewerage system on the verge of collapse. About one-third of the 1.4 million residents are cut off from the water supply and 75 per cent of Gaza is currently without electricity, including the Shifa Hospital in Gaza City, the strip's largest, the report said. Shifa has backup generators.

The Palestinian telephone company Paltel warned that Gaza's communications network has been extensively damaged by the Israeli strikes and is on the verge of collapse. The company added that three of its technicians had been killed and many injured in the fighting while trying to repair the network.

Adding to the shortages, last week's bombing further battered Gaza's infrastructure, making many feel that the situation they thought couldn't get any worse, had.

"When there was a siege, we kept talking about a catastrophe," said Hatem Shurrab, 24, of Gaza City. "But then the air strikes started, and now we don't even know what word to use. There's no word in the dictionary that can describe the situation we are in."

- AP