Israel is facing mounting pressure to agree a ceasefire after an attack on a UN school in Gaza left 43 dead and around 100 wounded.
On Tuesday, the Israelis launched an attack on the school which is run by the United Nations Relief and Works Agency (Unrwa) in the northern town of Jabaliya.
Hundreds of Palestinian civilians were sheltering in the building, which is the third school in Gaza to come under Israeli fire in 24 hours.
Doctors said all the dead were either people sheltering in the school or residents of the nearby Jabaliya refugee camp.
Around 15,000 Palestinians have had to flee their homes because of the fighting amid concern there are no safe havens in Gaza.
John Ging, director of operations in Gaza for Unrwa, said three artillery shells landed near the school where people were taking shelter from the Israeli offensive that is now entering its 12th day.
'Casualties inevitable'
Ging said Unrwa regularly provided the Israeli army with exact geographical co-ordinates of its facilities and the school was in a built-up area.
"Of course, it was entirely inevitable if artillery shells landed in the area there would be a high number of casualties," he said.
In an interview with Al Jazeera, Ging called for the Israelis to allow an international investigation into the incident.
When asked if Tel Aviv would allow such an investigation, Mark Regev, spokesman for the Israeli prime minister, said Israel had already held an "initial investigation" which found that troops had returned fire from the UN building.
Regev accused Hamas, the Palestinian faction that controls the Gaza Strip, of committing a "war crime" by using those sheltering in the UN school as "a human shield".
However, the attack has provoked strong international condemnation with Ban-Ki-moon, secretary-general of the United Nations, branding the incident "unacceptable".
"These attacks by Israeli military forces which endanger UN facilities acting as places of refuge are totally unacceptable, and should not be repeated.
In addition, Hugo Chavez, the Venezuelan president, expelled the Israeli ambassador in Caracas in protest over the Israeli military operation in Gaza.
'Humanitarian corridor'
At least 680 Palestinians have been killed in Gaza and nearly 3,075 wounded since the war began on December 27. Seven Israeli soldiers and three civilians have died in the same period.
Heba, a Gaza resident and mother of two, told Al Jazeera there was no place left in Gaza that can be considered safe.
"What happened in the school was a hugely offensive and inhumane thing. We never expected that people who sought refuge in a UN building would be attacked and killed," she said.
Randa Seniora, from the Independent Commission on Human Rights, told Al Jazeera: "What is happening in Gaza are crimes against humanity.
"Israel cannot claim, as an occupying authority, that it is acting in self defence because simply it is considered a war crime to create harm and damage among civilian populations."