On Tuesday, the Israelis launched an attack on a school run by the United Nations Relief and Works Agency (Unrwa) in the northern town of Jabaliya.
The attack left 43 Palestinians dead and around 100 wounded.
Doctors said all the dead were either people sheltering in the school or residents of the nearby Jabaliya refugee camp.
'No safe havens'
Around 15,000 Palestinians have had to flee the fighting amid concern there are no safe havens in Gaza.
Israel says missiles were fired from the UN building and that their troops were simply returning fire on Palestinian fighters using the school as cover.
However, Michel Abdel Massih QC, a London-based international human rights lawyer, called for an independent investigation.
"If [Israeli] claims are to be tested properly we need an independent tribunal. The International Criminal Court was set up to deal with these issues, so there is a mechanism for the UN security council to refer the case for investigation," he told Al Jazeera.
Massih compared the 680 Palestinian casualties to 10 Israeli deaths and underlined that proportionality of use of force is central to international law.
"From an international law point of view, there is at very least a case to answer against the planners and those who executed those orders," he said.
John Ging, director of operations in Gaza for Unrwa, said they regularly provide the Israeli army with exact geographical co-ordinates of its facilities.
He also called for the Israelis to allow an international investigation into the incident.
'War crime' allegation
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".
|War crimes: Key facts
The International Criminal Court says war crimes are defined as the violation of the laws and customs of war.
The murder, ill-treatment or deportation of civilian residents of an occupied territory.
The extensive destruction of cities, towns and villages, and any devastation not justified by military necessity.
The incident has provoked strong international condemnation with Ban-Ki-moon, secretary-general of the United Nations, branding both the incident and Hamas rocket attacks "unacceptable".
At least 683 Palestinians have been killed in Gaza and nearly 3,085 wounded since the war began on December 27. Seven Israeli soldiers and three civilians have died in the same period.
Following increasing pressure to agree a ceasefire as the humanitarian situation in Gaza deteriorates further, Israel announced on Wednesday it would set up "humanitarian corridor" to ship vital supplies to the Strip.
From Wednesday, Israel says it will halt military operations near Gaza City for three hours daily to let aid through.
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."
While earlier attempts to agree a ceasefire resolution at the UN have so far floundered, a French-Egypt proposal appeared to be gaining some support early on Wednesday.
The deal, which could include stationing international monitors at the Egyptian-Gaza border, has received qualified backing from the US and support from Mahmoud Abbas, the Palestinian president and leader of Hamas-rival Fatah.
Despite this, Jacky Rowland, Al Jazeera’s correspondent in southern Israel, said the Israelis are thought to be considering stepping up their assault.
The Israeli security cabinet is due to meet later on Wednesday to debate whether troops should storm Gaza's urban centres, our correspondent reported.