UNITED NATIONS - A UN agency that is a lifeline for 750,000 Palestinian refugees in Gaza suspended food delivery operations on Thursday after Israeli airstrikes killed one of its drivers and injured another. The UN Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) said the clearly-marked convoy carrying a UN flag and picking up supplies at the Erez crossing into Gaza had been coordinated with Israeli liaison officers, who gave the greenlight. "This is heartbreaking ... a very, very difficult decision for us to take" to suspend food delivery, UNRWA director of operations in Gaza John Ging told a news conference at the UN headquarters in New York, speaking by video link from Gaza. "The people here are in a dreadful state and really need our help at this point but we have also a responsibility to our staff, and eager as they are, and believe me they are, we cannot fly in the face of the security situation." While UNRWA is perfectly prepared for reasonable risks in a conflict zone, "added to those is the fact that we cannot rely on firm commitments given by the Israeli side," Ging said. The movement of all staff would be suspended until UNRWA could be assured of their safety, he said. Meanwhile, UN Under-Secretary-General for Humanitarian Affairs John Holmes, speaking at the same news conference, said: "We are seeking those assurances ... We're talking very urgently about how we can make better arrangements, more reliable arrangements to avoid these incidents on the ground which we simply cannot see repeated. Otherwise we simply won't be able to do our job." Ging said Israel was investigating the Erez incident, which came two days after Israeli shelling near an UNRWA school killed 40 people and injured more than 100. Israel said it was returning fire from the area of the school. UN officials stressed there were no Hamas or other militants inside the school. A second equally coordinated and marked UN medical convoy on its way to fetch the body of an UNRWA staffer killed in an earlier bombardment came under light arms fire in Gaza City on Thursday. Ging said he had received no credible explanation from Israel as to how "the green light turned into three rounds" of light arms fire in the incident, which occurred during Israel's stated daily three-hour lull in fighting. Asked how he could be sure that Israel was responsible for the incidents, Ging said the Israelis had not so far disputed the cases. Holmes said: "The headlines are that we desperately need a full and immediate ceasefire in Gaza to enable the civilian population to be protected and helped and that need is becoming more desperate by the hour." He stressed that most of the Gaza Strip was without power, although some fuel was getting through, and 20,000 people were in makeshift UN shelters. The Israeli lull, which the UN has called totally insufficient, allowed residents to get food supplies, access medical services, get to the dead and wounded in buildings. At the same time, rocket fire into Israel continued. Holmes cited credible Palestinian reports as saying the death toll had now reached 758, of whom 257 were children and 56 women, with 3,100 wounded including 1,080children and 452 women. He referred to "absolutely horrifying" accounts that the International Red Cross had reached bombed houses in Zaitoun, finding 12 dead people and four children alive next to their dead mother in one, and 15 wounded in another, noting what was particularly shocking was that a nearby Israeli team must have been aware of the wounded, yet did nothing. While UNRWA has suspended its supplies for the 750,000 people classified as refugees, a further 250,000 of Gaza's 1.5 million inhabitants receive food aid from the UN World Food Program (WFP), which has not yet taken a decision on its operations, UN officials said. Also on Thursday, UN Secretary-General Ban Ki-moon condemned the incidents, once again calling for an immediate ceasefire. In a statement issued by his spokesperson, he noted that four UNRWA local staff had been killed since the conflict started on Dec. 27. UN Children's Fund (UNICEF) executive director Ann Veneman voiced deep concern that the ongoing violence had forced UNRWA to suspend operations. "This can only deepen an already critical humanitarian situation and put children at even greater risk of death or permanent damage," she said in a statement. "The distribution of food, water, fuel and medicine should not be impeded. UNICEF calls on all parties to the conflict to take every measure to protect children. It is only with an end to the conflict that children's rights can be fully respected. "In the interim, safe spaces and unimpeded humanitarian access must be established in Gaza urgently to ensure that children have access to regular life-saving supplies and support," the statement said. In a related development, UN High Commissioner for Refugees Antonio Guterres told an open Security Council meeting on Thursday morning that, with Gaza's borders closed, the civilian population cannot flee to safety elsewhere.