The shell slammed into the school in northern Gaza as the conflict between Israel and Hamas entered its 23rd day, with international efforts to bring an end to the bloodshed continuing apace.
The conflict has killed around 1,260 people, including at least 32 on Wednesday morning, with concern growing over the high civilian death toll, especially among children.
On Wednesday morning, the Israeli shell slammed into the UN school being used as a shelter for those displaced by fighting, killing 20 people, emergency services spokesman Ashraf al-Qudra said. A UN official put the death toll at 16.
The shell hit the Palestinian refugee agency (UNRWA) girls' school in Jabalia refugee camp. The incident occurred after 5:30 am (0230 GMT), and a couple of hours after Israeli tanks had begun heavy shelling in the area. An AFP correspondent said that at least one shell had hit the school – the outer wall of the complex was damaged by shellfire, and in a bombed-out classroom, people were picking body parts off a blood-soaked floor. A number of donkeys killed by the shelling lay outside.
Displaced Palestinians who had already had to leave their homes quickly gathered belongings and fled the building. The shelling came as a Palestinian delegation prepared for a trip to Cairo to discuss a temporary humanitarian ceasefire.
The West Bank-headquartered Palestine Liberation Organisation (PLO), which has been at odds with Hamas for years, said it had garnered the Islamist movement's support for a 24-hour truce, but did not say when that was due to start.
Israel's government had no comment on the proposal.
Palestinian president Mahmud Abbas "was in touch with (Hamas chief Khaled) Meshaal yesterday and today. He proposed the 24 hour truce, Meshaal and Hamas agreed", senior Palestinian official Nabil Shaath told AFP on Tuesday.
International efforts have focused on getting Israel and Hamas to agree to a temporary humanitarian ceasefire, and then extending that truce for a longer period while they intensify efforts to end hostilities altogether. But apart from a fragile halt on Saturday after which hostilities picked up with renewed vigour, efforts have failed.
Children paying the price
Heavy Israeli bombardment of Gaza overnight Monday to Tuesday killed scores of Palestinians after they marked the beginning of the Muslim Eid festival, medics said.
Israel's aerial, naval and artillery shellings have killed some 1,262 Palestinians in the bloodiest conflict in six years, and the vast majority of Palestinian dead are civilians, according to the UN.
Hamas fire, including thousands of rockets launched at Israel, has killed 53 soldiers and three civilians inside the Jewish state.
Rights groups have expressed alarm at the number of children victims.
The more than 240 children who have died represent at least 29% of civilian casualties, the United Nations Children's Fund, UNICEF, said in a statement, adding that another six children in Israel had been wounded from Gaza rocket fire.
"We see children killed, injured, mutilated and burnt, in addition to being terrified to their core. The consequences run much deeper than previous flare-ups" in Gaza, UNICEF's Gaza field office chief Pernille Ironside said.
Since the war began on July 8, when Israel launched a campaign to stop rocket fire from Gaza and destroy attack tunnels, a series of concerted international efforts to bring a truce have fallen flat.
US Secretary of State John Kerry said that Israeli Prime Minister Benjamin Netanyahu had asked for fresh help from America in trying to broker a ceasefire. "Last night we talked, and the prime minister talked to me about an idea and a possibility of a ceasefire. He raised it with me, as he has consistently," Kerry said Tuesday. The top US diplomat added that Netanyahu had said he "would embrace a ceasefire that permits Israel to protect itself against (Palestinian militants') tunnels."
PLO secretary general Yasser Abed Rabbo said after consultations with Hamas and Islamic Jihad, the two main militant groups in Gaza, that there was "willingness for a ceasefire and humanitarian truce for 24 hours".
Hamas said so far it had not agreed to any new truce and was waiting for Israel to show its hand first. "When we have an Israeli commitment... on a humanitarian truce, we will look into it but we will never declare a truce from our side while the occupation keeps killing our children," Hamas spokesman Sami Abu Zuhri said.
As both sides appeared more determined than ever to keep up the fighting, the hostilities have pushed more than 215,000 people to flee their homes in the overcrowded territory, according to the World Health Organization.