Israeli forces shelled areas deep inside Gaza City on Thursday, hitting the headquarters of the United Nations Relief and Works Agency and injuring at least three people in the compound, according to witnesses and United Nations officials.

A spokesman for the agency, Christopher Gunness, said that hundreds of Palestinians were sheltering in the compound at the time of the shelling and that there were five fully laden fuel vehicles at the site. He said two buildings were ablaze.

The United Nations compound was struck as Israeli ground forces pushed further into Gaza City and intensified its shelling of both outlying neighborhoods and central districts, sending thousands of panicked residents fleeing from their homes, witnesses said. Among other buildings hit in the center of Gaza City, the witnesses said, was one occupied by several media organizations, and at least two television cameramen were hospitalized.

The United Nations secretary general, Ban Ki-moon, expressed "strong protest and outrage" to Israel over the shelling of the United Nations compound. Mr. Ban, who was in Israel Thursday to promote a cease-fire, said that Israeli Defense Minister Ehud Barak had told him the strike on the compound was a “grave mistake.”

Mr. Gunness said that the Israelis had been provided with the GPS coordinates of all United Nations facilities in Gaza.

Earlier in Israel’s 20-day-old campaign against Hamas, Israeli mortar shells landed outside a United Nations school compound in Jabaliya, northern Gaza, killing 40 Palestinians, according to United Nations and hospital officials.

In that attack, the Israeli military said it was responding to mortars fired by Hamas militants from a yard next to the school compound, and that one of the shells it fired back fell off the mark.

As the fighting in Gaza entered its 20th day, the Israeli military would not give precise details of its latest ground operations in Gaza City, but a spokesman said that “fierce fighting” was under way “relatively deep inside Gaza.”

The military push may be aimed at stepping up pressure on Hamas as cease-fire talks in Egypt entered a pivotal stage.

Overnight, the military said, Israeli planes struck around 70 targets, including a mosque in the southern town of Rafah that it said was used to stockpile rockets, and several squads of gunmen. Within two hours on Thursday morning, militants in Gaza launched 15 rockets and mortars against Israel, the military said, a marked increase in fire compared to Wednesday when there were 16 launches during the entire day.

Palestinians arrived with injured relatives at Shifa Hospital in Gaza City on Thursday, some barefoot and in nightgowns. They told of intense Israeli shelling in several neighborhoods including the Sabra and Tufah districts. Two television cameramen arrived for treatment after the building housing the media offices was hit. They had been filming from a window, they said.

Residents of the Tel el-Hawa district in south-western Gaza City said Israeli shelling and shooting had gone on all night and that the local Al Quds Hospital was under fire.

As the Gaza death toll passed an estimated 1,000 people and concerns about the humanitarian situation inside Gaza grew, Egypt announced on Wednesday that it was making progress toward an interim cease-fire, with some officials predicting that one could be five to six days away. A senior Israeli defense official, Amos Gilad, arrived in Cairo on Thursday to continue the talks.

Also on Wednesday, nine Israeli human rights groups called for an investigation into whether Israeli officials had committed war crimes in Gaza. The groups say that tens of thousands of civilians in Gaza have nowhere to flee, the Gaza health system has collapsed, many people are without electricity and running water, and some are beyond the reach of rescue teams.

“This kind of fighting constitutes a blatant violation of the laws of warfare and raises the suspicion, which we ask be investigated, of the commission of war crimes,” the groups said in a news conference on the 19th day of the war.

The president of the International Committee of the Red Cross, Jakob Kellenberger, who spent Tuesday in Gaza City, agreed that the situation with civilians was dire but said that the principal hospital was making do with medical supplies, and that doctors, working around the clock, were mostly coping with the flow of the wounded.

“In general, they did not complain about the lack of equipment or material,” he said at a news conference in Jerusalem.

Hamas’s leaders met with Egyptian officials in Cairo and agreed in principle to a monitoring force in Gaza composed of Europeans to prevent weapons smuggling, said a senior Egyptian official. Israel’s defense minister, Ehud Barak, and his generals favor a temporary cease-fire of several days to a week, partly so that when President-elect Barack Obama is inaugurated next week it would be during a lull rather than in the middle of a battle, and his administration could offer its views on the next step, Israeli officials said.

The short-term cease-fire would, if successful, be followed by a negotiated yearlong truce, something that Egypt says Hamas favors if it includes an opening of commercial traffic into Gaza. But splits in Hamas exist between its leaders based in Syria and those in Gaza. The Gazans are more open to a weeklong break, while the leaders in Syria want something from Israel in return for holding fire.

Secretary General Ban Ki-moon of the United Nations arrived in Cairo Wednesday as part of a regional tour to press all parties to carry out a Security Council resolution calling for a cease-fire. He met with President Hosni Mubarak and then issued a plea for peace.

President Bashar al-Assad of Syria also called for a cease-fire, saying in an interview with the BBC that the effects of war could be more dangerous than war itself, “sowing seeds of extremism and terror around the region.”

Osama bin Laden, the leader of Al Qaeda, in a taped audio message, called on Muslims everywhere to fight Israel in a holy war.

The Israeli human rights groups that called for an investigation said that while they believed that it was legitimate for Israel to bomb military installations, it was a violation of international law to hit civilian sites and government buildings that contained no weapons.

The groups included the Israel section of Amnesty International, B’Tselem, Gisha and Physicians for Human Rights-Israel. Israeli Jews are firmly behind the government’s conduct of the war, with the human rights groups representing a small minority.

Mr. Kellenberger of the Red Cross said that Israel had facilitated his trip to Gaza and added that he had seen no evidence of the use of white phosphorus, an obscurant used in military conflicts that can be dangerous for civilians under certain circumstances. Palestinians say Israel is using it in Gaza.

Last week, the Red Cross issued an unusually harsh condemnation of Israel for refusing to allow its personnel into Gaza to rescue people trapped in battle. On Wednesday, Mr. Kellenberger said that although the situation remained critical, rescue missions had not been entirely shut down. The organization rescued 100 people trapped in Jabaliya, north of Gaza City, on Tuesday.