The United Nations suspended all activities in the Gaza Strip Thursday, accusing Israeli soldiers of firing on a marked UN vehicle during a three-hour humanitarian cease-fire initiated by Israel.

The international Red Cross also complained that Israel was imposing "unacceptable" delays on its workers and said it was restricting its movement inside Gaza during Israel's ongoing offensive against Hamas.

Just two days after a tank shell killed about 40 Palestinians, mostly civilians, taking refugee in a UN school in the Jabaliya refugee camp, a spokesman for the UN in Gaza accused the Israeli army of ignoring efforts to coordinate movement amid the fighting.

The Red Cross complained that Israel is hindering access to civilians.

Because the UN is one of the biggest providers of food and medical relief, halting shipments to Gaza will accelerate the worsening conditions in the besieged coastal enclave of 1.5 million Palestinians. Israel is facing growing criticism due to the poor humanitarian conditions inside Gaza, and a burgeoning crisis would likely lead to more international pressure for a withdrawal of its forces.

"The IDF knows all of our movements, and coordinates. Despite that we are still coming under attack," says Chris Gunness, a spokesman for the UN Relief and Works Agency (UNRWA). "Until the [Israeli Defense Forces] can guarantee the safety and security of our staff, everything is suspended."

Palestinians and human rights groups say that international aid is the only bulwark preventing a full-blown humanitarian crisis in Gaza. About half of the population receives food aid, according to the Associated Press. UNRWA supplies account for about half of the daily truck containers of relief supplies crossing from Israel into Gaza.

Israel's military said that it was investigating the allegations made by the UN and the Red Cross. The UN also demanded an investigation into Tuesday's shelling near the UN school.

Israel says that militants fired rockets from the area of the school and then ran into the crowd of civilians to take cover.

An official for the Israeli military's civilian liaison says that the army hopes to renew cooperation with the UN.

"We work closely with these organizations," says Peter Lerner. "We view their operations as necessary at all times."

International efforts toward a cease-fire continued to make progress Thursday. Diplomats meeting at the UN in New York are reportedly nearing a decision on a UN Security Council resolution that calls for an end to the Israel-Hamas fighting. The United States, Britain, and France dropped their opposition to language calling for an immediate cease-fire, the AP reported.

More than 700 Palestinians have been killed and thousands wounded in nearly two weeks of fighting. Four Israelis have been killed by cross-border rockets attack and another seven soldiers have died in the fighting, four of whom were killed by friendly fire.

The UN vehicle that was struck by Israeli gunfire Thursday came under small-arms fire from two directions in the northern Gaza Strip, despite having coordinated with the Israeli army.

Mr. Gunness says that UN personnel, institutions, and convoys have come under fire repeatedly throughout the 13-day conflict.

With a staff of about 9,000, UNRWA operates food distribution centers, medical facilities, and a network of schools in Gaza. The food shipments accounted for roughly half of the total supply entering Gaza this week.

Other relief groups have complained of bureaucratic delays in sending supplies to Gaza.

If the food contributions from relief groups were cut off, the real situation imposed by Israel would be revealed, says Miri Weingarten, a spokeswoman for Physicians for Human Rights.

"The enormous amounts of international aid shipped in is holding up the line of subsistence," she says. "Basically the only one capable of preventing disaster is the government of Israel if the shipments don't resume again."