One of the strongest voices inside the White House urging an end to civilian casualties in Gaza is the person closest to the president: Jill Biden.

At a meeting with Muslim community members at the White House on Tuesday evening, one guest told the president that his wife had disapproved of him coming to the meeting because of Mr. Biden’s support for Israel in its war against Hamas.

Mr. Biden replied that he understood. The first lady, he said, had been urging him to “Stop it, stop it now,” according to an attendee who heard his remarks.

Salima Suswell, the founder of the Black Muslim Leadership Council, recounted the scene in an interview, adding that she had scribbled down the president’s statements because it was so striking to hear that the first lady felt strongly about the conflict.

“He said she said, ‘Stop it, stop it now, Joe,’” Ms. Suswell said.

Asked about the president’s remarks, White House officials on Wednesday said that there was no daylight between Mr. Biden and the first lady on the conflict and that the president was as outraged by the civilian casualties as his wife has been. The officials said the first lady was not calling for Israel to end its efforts against Hamas.

“Just like the president, the first lady is heartbroken over the attacks on aid workers and the ongoing loss of innocent lives in Gaza,” Elizabeth Alexander, the first lady’s communications director, said in a statement. “They both want Israel to do more to protect civilians.”

The first lady is not alone in urging her husband on. A number of Mr. Biden’s closest allies, including Senator Chris Coons, Democrat of Delaware, have pressed the president to do more to increase humanitarian aid to Gaza and bring the war to a close, including by supporting restrictions on military aid to the Israelis.

Mr. Biden has faced growing alarm among other Democrats about his support for the war, as well as letters of internal dissent, including objections from officials at some 40 government agencies.

But the first lady occupies the most influential space within Mr. Biden’s inner circle and is one of the few people who offer him an unvarnished opinion on matters of policy and politics.

Dr. Biden has been against American involvement in overseas conflicts in the past, partly because her son Beau Biden had enlisted in the Delaware Army National Guard in 2003 and was deployed to Iraq in 2008. She supported her husband’s decision to pull American troops out of Afghanistan, despite the deadly, chaotic effort that followed.

“I think he trusts my intuition as a spouse,” she told The New York Times in an interview in 2021, “not as a policy person or an adviser.”

A person who attended a White House function during the winter of 2022 was surprised at the first lady’s emotional response to someone praising the legacy of President George W. Bush: “He sent my son to war,” Dr. Biden replied angrily. “He sent my son to war.”

The conflict in Gaza is different. American troops are not on the ground, but Mr. Biden’s staunch support of Israel over the past six months has put him squarely at odds with the majority of Americans who now oppose the Israeli campaign in Gaza, according to recent polling. Some Democratic voters in battleground states, including Michigan and Wisconsin, are warning the president that they will withdraw support for him should he continue to stand by Israel.

On Tuesday, Mr. Biden’s remarks to the group came as he released a statement saying that he was outraged by the deaths of seven aid workers with the World Central Kitchen killed by Israeli airstrikes on Monday night. But so far there is no indication that the first lady’s private urgings have prodded the president to change his policy toward Israel.

Mr. Biden is scheduled to speak with Prime Minister Benjamin Netanyahu of Israel on Thursday, according to a senior administration official. That official, who was not authorized to speak publicly, said that Mr. Biden’s anger and frustration had hit a peak in recent weeks.

Mr. Biden’s steadfast support for Israel has made it more difficult for the first lady to do the kind of campaigning she has done over decades of marriage. Like her husband, Dr. Biden has been interrupted at several campaign events by people protesting American involvement in the conflict.

In Arizona in March, a Women for Biden event headlined by Dr. Biden was interrupted by protesters who demanded to know why the Bidens were supporting “genocide” in Gaza.

At other events that month, in Wisconsin and Vermont, people gathered outside of Democratic Party offices and homes of donors to protest American support for the war. A keynote address on March 23 that Dr. Biden delivered at a Human Rights Campaign dinner in Los Angeles was interrupted by a group of people who jumped up from their seats and began shouting “cease-fire now.”

In that event, the first lady stayed quiet while the protesters were removed from the room amid chants of “four more years” by supporters. After they were gone, she resumed her speech.