Israel’s retaliation to the Hamas attacks have killed 18,000, Gaza officials say, wounded 50,000 and created a humanitarian crisis. Biden’s remarks opened a new window into his blunt private conversations with Netanyahu, with whom he has had deep disagreements for decades.

Biden alluded to a private conversation in which the Israeli leader said: “‘You carpet bombed Germany, you dropped the atom bomb, a lot of civilians died.’”

Biden said he responded: “Yeah, that’s why all these institutions were set up after World War II to see to it that it didn’t happen again … don’t make the same mistakes we made in 9/11. There’s no reason why we had to be in a war in Afghanistan.”

Biden, who often speaks off the cuff at his fundraising events, spoke at a Washington hotel to a crowd of about a hundred that included a number of Jewish attendees. He was introduced by a long-time leader of the American Israel Public Affairs Committee, a pro-Israel lobby.

The sharp comments came as Biden’s national security adviser, Jake Sullivan, prepares to travel to Israel for talks with the Israeli war cabinet.

Netanyahu said in a statement on Tuesday that Israel had received “full backing” from the US for its ground incursion into Gaza and that Washington had blocked “international pressure to stop the war”.

But he added: “There is disagreement about ‘the day after Hamas’, and I hope that we will reach agreement here as well.”

At the fundraiser, Biden specifically mentioned Israel’s far-right politician Itamar Ben-Gvir, who is Israel’s national security minister, and said “this is the most conservative government in Israel’s history”.

“[Netanyahu] has to change this government. This government in Israel is making it very difficult,” Biden said.

He also said that ultimately Israel “can’t say no” to a Palestinian state, which Israeli hardliners oppose.

Biden said: “We have an opportunity to begin to unite the region … and they still want to do it. But we have to make sure that Bibi [Netanyahu] understands that he’s got to make some moves to strengthen … You cannot say no Palestinian state … That’s going to be the hard part.”

Netanyahu also said in Tuesday’s statement he would “not allow Israel to repeat the mistake of Oslo”, the 1990s peace accords that created the Palestinian Authority as part of negotiations for the creation of a potential Palestinian state in the West Bank, Gaza and East Jerusalem.

Washington has said it envisions an eventual return by the PA to Gaza, which Hamas seized from the West Bank-based body in 2007.

“I will not allow the entry into Gaza of those who educate terrorism, support terrorism and finance terrorism,” Netanyahu said. The PA denies such allegations.

Biden has expressed strong support for Israel’s military operation against Hamas militants in Gaza but he and his team have expressed growing concern about the death of Palestinian civilians.

Biden plans to meet on Wednesday at the White House with family members of Americans taken hostage by Hamas during its October 7 attack in which 1,200 people were killed, a White House official said.

Sullivan said on Tuesday that during his visit to Israel he will discuss with Israeli officials their timetable for the war in Gaza.

“The subject of how they are seeing the timetable of this war will certainly be on the agenda for my meetings,” said Sullivan, who is expected to travel later this week.

Sullivan blamed Hamas for the breakdown of a truce from November 24 to December 1 because the militants refused to release more hostages.

“Hamas to this day continues to hold women, elderly people, civilians in significant numbers. And yet still, it’s saying: ‘Hey, how about everybody just stops.’ So we believe that Israel has the right to defend itself,” he said.