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Israel has a responsibility under international law to protect civilians, as its forces expand their ground operations in Gaza, a US official said on Sunday.
Mr Sullivan said Hamas was hiding behind civilians but that does not lessen Israel’s “responsibility under international humanitarian law and the laws of war to do all in their power to protect the civilian population”, he said in an interview with ABC.
In another interview on CNN, Mr Sullivan said the Israeli army should do all it can to distinguish between civilians and military targets.
“What we believe is every hour, every day of this military operation, the IDF [Israeli military], the Israeli government should be taking every possible means available to them to distinguish between Hamas terrorists who are legitimate military targets and civilians who are not,” he said.
His comments came amid growing international outcry over Israel's air and land bombardment of the Gaza Strip that began on October 7, after Hamas gunmen attacked southern Israel, killing 1,400 people.
The backlash was magnified on Friday, after communication services were cut amid a step up in Israeli air strikes on the territory, including from Mr Biden's own Democratic party.
“We have to recognise that our credibility and our authority on the world stage is greatly diminished if we do not also call out this siege that Israel is launching on Gaza as violations of international law,” US Representative Pramila Jayapal, leader of the Congressional Progressive Caucus, said on NBC.
“We're being isolated in the rest of the world,” Ms Jayapal said.
At least 8,000 Palestinians have been killed, including more than 3,000 children, according to medical officials in Gaza, and the enclave's 2.3 million residents are facing a worsening humanitarian crisis as Israel has blocked the entry of water, food, medicine and fuel.
The US has vetoed two UN Security Council resolutions calling for a ceasefire, and has instead called for “humanitarian pauses” that would allow aid in.
“There have been deaths of thousands of Palestinian civilians in this conflict and that is an absolute tragedy,” Mr Sullivan told ABC.
“Those people did not deserve to die,” he said. “Those people deserve to live lives of peace and safety and dignity.
Israeli Prime Minister Benjamin Netanyahu has said Israel is intent on destroying Hamas and that the war, now in its third week, has now entered a new phase.
Mr Sullivan said President Joe Biden was due to speak to Mr Netanyahu later on Sunday with a view to holding “candid” talks.
The Biden administration is negotiating the entry of more humanitarian aid into the Gaza Strip, the exit of American citizens and the release of hostages.
So far only several dozen lorries carrying much-needed supplies have been able to enter Gaza through the Egyptian border.