Washington (AFP) - The Hamas attack on key US ally Israel has caused emotions to run high in a place better known for the steely exercise of power -- the White House.

President Joe Biden has given several strongly worded speeches pledging support for Israel since Saturday's assault by the Palestinian militant group, and at one point pounded on a lectern to make his point.

Emotion got the better of a senior White House official who choked up on live television as he discussed the attacks. Biden's national security advisor admitted the crisis was "personal for us."

The unusually open displays reflect deep anger in the White House over the attacks, which killed more than 1,200 people, as well as its long-standing ties with Israel.

In contrast, the White House's reaction to the fate of people in Gaza after more than 1,800 people were killed in a week of Israeli strikes has been more muted.

'Pain'

The 80-year-old Biden has long been a stalwart supporter of Israel and described this week how he had known "Bibi" -- Prime Minister Benjamin Netanyahu -- for half his lifetime.

Ties had cooled until recently due to a row between the two leaders over the Israeli government's judicial reforms.

But that disagreement has been forgotten.

"You OK kiddo?" Biden broke off to ask of one emotional participant during a roundtable for US Jewish Community leaders at the White House on Wednesday.

Introducing the event was Douglas Emhoff, the Jewish husband of Vice President Kamala Harris, who has led a series of outreach events amid fears of rising anti-Semitism in the United States in recent years.

One moment in particular showed the feelings animating Biden, who has said that he and his team are working constantly on the crisis.

The president brought his fist down hard on the podium as he described how he had taken all his children to the Dachau Nazi concentration camp to understand the horrors of the Holocaust.

"I wanted them to see," he said, raising his voice so it echoed round the cavernous Indian Treaty room at the White House.

Behind the scenes, White House officials spoke of the difficulty of seeing "brutal" images from the Hamas attacks.

The tension is compounded by the fact that Americans are among around 150 people taken hostage, while the administration is also on guard in case the conflict turns into a regional confrontation. 

On Friday, Biden spoke with the families of 14 Americans who have been missing since the Hamas attack. 

The White House did not release any details about the call, but Biden told CBS's "60 Minutes" that the families "have to know that the president of the United States of America cares deeply about what's happened to them, deeply."

"And we're going to do everything in our power to get them home if we can find them."

'Human cost'

The White House's often raucous press briefings have been more somber since the attacks, with stern faces animated by occasional flashes of emotion.

Normally known for his calm demeanor, Biden's National Security Advisor Jake Sullivan told reporters on Tuesday that it had been a "deeply emotional time for all of us."

"I hear the pain of my counterparts when I talk to them," Sullivan, 46, said at the famed blue podium. "This is not just about policy or strategy -- this is personal for us."

The strain also showed when National Security Council spokesman John Kirby had to pause to recollect himself while talking on CNN at the start of the week.

"Sorry, excuse me," the former navy rear admiral said, his voice catching with emotion. "It's very difficult to look at these images -- the human cost."

But the White House is now having to steel itself for a crisis that could upend the Middle East as Israel gears up for an expected ground invasion of Gaza.

Left-wing lawmakers from Biden's Democratic Party on Friday urged him to work to avoid a humanitarian disaster in Gaza and to insist international law is followed as Israel pushes on with its fierce bombardment.

"We can't lose sight of the fact that the overwhelming majority of Palestinians had nothing to do with Hamas and Hamas's appalling attacks, and they're suffering as a result as well," Biden said Friday.

Did the United States believe Israel was violating the laws of war? The question was asked of Kirby at a White House briefing.

"It's always on the president's mind -- the protection of civilian life," he replied.