Israel's offensive in Gaza has forced as many as 90,000 Gazans to abandon their homes, and thousands more may soon have to flee in search of safety, humanitarian aid organizations said Monday.

For civilians caught in the crossfire between Hamas militants and Israel's military, however, there's no escape to safety abroad and no sure sanctuary in Gaza. Some families have had to move repeatedly to escape the violence.

In the 17 days since Israel began pummeling Hamas militants in the Gaza Strip, Mohammed al-Sultan's family has moved four times, from house to house, shelter to shelter, having left all their possessions behind. Now they're among hundreds of displaced Palestinians who are sleeping in cold concrete classrooms in a schoolhouse that's serving as a temporary shelter in Gaza City.

"They have no place to hide, and no place to run," said U.N. Secretary-General Ban Ki-moon, who announced that he would visit the region this week to push for a cease-fire that the U.N. Security Council has ordered but that Israel and Hamas have ignored.

Senior Israeli officials said they're close to achieving the objectives they set for the war: stopping Hamas militants from firing rockets into Israel and preventing the group from re-arming itself. Relief officials and human rights groups, however, said the Israeli military failed to plan for the safety of civilians in one of the most densely populated patches of the Middle East.

When Maj. Avital Leibovich, an Israeli military spokeswoman, was asked at a news conference on Sunday where civilians would be safe in Gaza, she suggested that refugees could find shelter in the razed Israeli settlements on Gaza's northern border with Israel. That would require refugees to go through Israeli-controlled military areas, however.

Israeli officials said that Hamas operates from the vicinity of homes, schools and mosques, making fighting in civilian areas unavoidable. Of the more than 900 Palestinians who have died in the conflict, about half were civilians, according to Gaza medical officials.

"They (Israel) knew this operation was coming, they knew what the geography and demographics of Gaza are like, and apparently they didn't plan for the humanitarian needs," said Fred Abrahams, a senior researcher for Human Rights Watch, a New York-based advocacy group.

So 1.5 million Gazans remain bottled up inside their tiny coastal strip - at about 140 square miles, roughly twice the size of the District of Columbia but three times as populous.

Al-Sultan's family has been on the run since the early days of the Israeli offensive. On the morning of Dec. 28, he was sitting with his wife and two daughters in the garden outside their home in the Salateen section of Gaza City when an Israeli airstrike hit nearby, jolting them "like a volcanic eruption" and sending shards of shrapnel into his back, he said.

The bespectacled 60-year-old went to a hospital, and his family moved in with his brother, but four days later a night of heavy bombardment forced them to move again, to the home of a friend in a neighborhood close to the beach. They moved so fast they carried nothing with them, not even al-Sultan's ID card.

"Four or five days and the bullets reached us again," al Sultan said. They moved to a U.N.-operated shelter, but it was overflowing with people, so when they got word over the weekend that another shelter was open at the Salaheedeen school in Gaza City, they moved for the fourth time in two weeks.

And perhaps not the last, he said.

The U.N. has opened 36 temporary shelters housing more than 28,000 people, but two of those have come under fire. Relief officials said that tens of thousands more displaced Palestinians are on their own, staying with family members or friends.

In the shelters, many of them in school buildings, most food comes from Palestinian or international charity groups. Women and girls wash clothes in buckets and hang them to dry anywhere they can find. Men haul desks into courtyards and sit idly or huddle in chilly classrooms to pass the time.

Human rights experts said the plight of displaced Palestinians is particularly severe because Gaza is completely sealed off from its neighbors, Israel and Egypt.

Egyptian President Hosni Mubarak has allowed in scores of wounded Gazans to receive medical care, but despite public pressure he hasn't agreed to host a large refugee population from such a volatile region.

"Gaza is truly exceptional," said Rupert Colville, a spokesman for the U.N. High Commissioner for Human Rights. "The way this conflict is unfolding, they clearly could be at risk wherever they are."

The U.N. has said that Egypt and Israel could be in violation of international refugee laws that give people in war zones the right to seek asylum in foreign countries.

"Normally people can at least dream of going somewhere safe," Colville said. "Gazans can't."

Even Gaza's so-called safe areas aren't safe. In what international humanitarian groups describe as one of the deadliest incidents of the conflict, an Israeli artillery strike on Hamas targets last week landed near a school that the U.N. had opened as a temporary shelter for displaced families, reportedly killing more than 40 people.

Israeli military officials said they're investigating the incident but that their ground forces were merely returning fire from Hamas fighters operating from near the school. U.N. officials didn't dispute Israel's contention that militants sometimes use their shelters as cover, but they've called for an independent inquiry.

The tensions over the war reverberated into Israeli politics Monday when Israel's election committee voted overwhelmingly to ban the nation's two Arab parties from running in next month's national election.

Ultranationalist Israeli lawmakers proposed the ban. They accused their Arab colleagues of betraying Israel and supporting terrorism because some of them have visited Israel's enemy neighbors, Syria and Lebanon. Arab lawmakers vowed to appeal the ruling.

Despite fiery rhetoric from Hamas leaders who have vowed to keep fighting, Israeli officials continued to declare that they had dealt a substantial blow to Hamas's infrastructure in Gaza. Israel's security Cabinet reportedly is divided between ending the conflict and sending ground troops deeper into urban areas.

"We're beginning to enter the end game," said government spokesman Mark Regev. "We're not there yet."