The deadly attacks by Hamas on Israeli civilians and the devastating Israeli airstrikes and blockade of Gaza have raised accusations among international legal experts that both sides were violating international law. A United Nations Commission of Inquiry said it has been “collecting and preserving evidence of war crimes committed by all sides” since the violence started last week. That evidence could be added to an investigation by the International Criminal Court into possible war crimes committed by Israel and Hamas in past conflicts. “Intentional targeting of civilians and civilian objects without a military necessary reason to do so is a war crime, period,” said David Crane, an American international law expert and the founding chief prosecutor of the United Nations’ Special Court for Sierra Leone. “And that’s a standard that both sides are held to under international law.” Even Israel’s staunchest ally has sounded a note of caution. U.S. President Joe Biden, at a meeting with Jewish leaders Wednesday, said he told Israeli Prime Minister Benjamin Netanyahu “that it is really important that Israel, with all the anger and frustration and just — I don’t know how to explain it — that exists is that they operate by the rules of war — the rules of war. And there are rules of war.” DID HAMAS COMMIT WAR CRIMES? After breaking through Israel’s security barrier early Saturday morning, Hamas militants gunned down entire families, including women and young children, in border communities around the Gaza Strip. Israel’s health service said it extricated the bodies of over a hundred community members from Kibbutz Be’eri. Militants attacked the Tribe of Nova music festival, gunning down people as they desperately sought refuge. The attacks killed more than 1,300 people in Israel, including 247 soldiers — a toll unseen in Israel for decades. Omar Shakir, Israel and Palestine Director of Human Rights Watch, pointed to Hamas “shooting civilians en masse, taking hostages, including women and children — undeniably grave abuses of international law, for which there’s no justification.” In an analysis published on the international law website Opinio Juris, Cornell Law School professor Jens David Ohlin wrote that the Hamas attacks amount to war crimes and crimes against humanity under the International Criminal Court’s founding Rome Statute. Rights group Amnesty International called for accountability. “Massacring civilians is a war crime and there can be no justification for these reprehensible attacks,” said Agnès Callamard, Amnesty International’s secretary-general. “These crimes must be investigated as part of the International Criminal Court’s ongoing investigation into crimes committed by all parties in the current conflict,” Callamard said. IS ISRAEL’S MILITARY RESPONSE LEGAL? The Israeli military has pulverized large parts of the Hamas -ruled Gaza Strip with airstrikes and blocked deliveries of food, water, fuel and electricity ahead of a possible ground invasion. The bombardment already has killed about 1,800 people in Gaza, including U.N. workers, paramedics and journalists. Experts say the blockade, which is hitting the territory's more than 2 million residents, violates international law. "Collective punishment is a war crime. Israel is doing that by cutting electricity, water, food, blocking aid from entering the Gaza Strip,” Shakir said. Early Friday, Israel’s military directed the evacuation of some 1 million civilians living in the northern Gaza Strip ahead of a feared Israel ground offensive. Hamas called on residents to remain in their homes. The International Committee of the Red Cross said the order to leave along with the siege "are not compatible with international humanitarian law.” Jan Egeland, secretary-general of the Norwegian Refugee Council, also called the order illegal. It is "not an evacuation opportunity, it’s an order to relocate. Under humanitarian law, it’s called forcible transfer of populations, and it’s a war crime,” he said. Israel has also faced criticism for its widespread airstrikes razing large areas of Gaza since the Hamas attacks. But Crane said that Hamas’ base in the densely populated area makes any Israeli military action extremely difficult. “They’re almost in an impossible situation. Every time they fire an artillery piece, an aircraft fires missiles and stuff at a legitimate target, they’re going to collaterally kill civilians,” he said. The Israeli military has “this challenge where you have one of the most densely populated places on Earth where you have a combatant hiding behind and firing from those positions, using the civilians as human shields,” Crane said. Many in Israel’s defense establishment have pledged to fight until every trace of militancy is gone from the territory — even if it means wreaking mass havoc on the besieged strip’s civilian population. But Israel's relentless airstrikes could come under scrutiny, both because of the heavy civilian death toll and heavy damage to civilian infrastructure. “We’re seeing reports of entire neighborhoods, blocks that are reduced to rubble. Certainly that would appear to be, you know, war crimes as well,” Shakir said. “We’ve seen attacks that have affected hospitals and other areas that are entitled to protection.” The Israeli army says it follows international legal norms and strikes only legitimate military targets. “The most pleasant way not to cause any harm to anyone is not to do anything,” said retired Israeli general Giora Eiland. “But Israel has to fight. And how do you fight? You have to bomb them. Or you do nothing. If civilians decide to stay on the streets of Gaza, there will be much more civilian casualties.” CAN THE ICC GET INVOLVED? While Israel is not one of the court’s 123 member states, ICC judges have ruled that the Palestinians are and that the court has jurisdiction over territories occupied by Israel in the 1967 Mideast war. The ICC prosecution office's ongoing investigation — spurred by the last major conflict in Gaza — can analyze war crimes allegations from the latest war. But Israel does not recognize the court's jurisdiction and the ICC does not have a police force to execute arrest warrants. ___ Frankel reported from Jerusalem. Read More A father worries for his missing child: 'My daughter didn't go to war. She just went to dance' US cities boost security as fears spread over Israel-Hamas war despite lack of credible threats BBC journalist ‘stopped and assaulted’ by Israeli police FACT FOCUS: Misinformation about the Israel-Hamas war is flooding social media. Here are the facts Israeli shelling along Lebanon border kills 1 journalist, wounds 6 In Israel's call for mass evacuation, Palestinians hear echoes of their original catastrophic exodus