Biden urged Israelis not to be consumed by rage and said the vast majority of Palestinians were not affiliated with Hamas. The Palestinian people are suffering as well, he said.
In remarks after meeting Israeli leaders, Biden said he would ask Congress for an "unprecedented" aid package this week.
The president made reference to the Nazi Holocaust of World War Two when saying that Israel had the backing of its friends.
"We will not stand by and do nothing again. Not today, not tomorrow, not ever," he said.
The United States has urged Israel to allow humanitarian aid into Gaza.
Earlier he said he had asked Israeli Prime Minister Benjamin Netanyahu tough questions during their meeting on Wednesday in Tel Aviv, where they also discussed humanitarian needs, security assistance and information on unaccounted Americans.
"I asked tough questions as a friend of Israel. We will continue to deter any actor wanting to widen this conflict." Biden said in a post on X, formerly known as Twitter.
(Reporting by Steve Holland; additional reporting by Doina Chiacu and Jeff Mason; Editing by Howard Goller)
