WASHINGTON — President Joe Biden denounced Hamas on Tuesday, calling the attack the Palestinian militant group launched against Israel "an act of sheer evil," and stressed US support for Israel as it mourns the killing of more than 1,000 people.

The president also voiced concern for Americans being held hostage by Hamas, an Iranian-backed Islamist group, whose stated purpose, he said, is to "kill Jews." He called for Israel to follow the "law of war" in its response.

"There are moments in this life, and I mean this literally, when the pure unadulterated evil is unleashed on this world. The people of Israel lived through one such moment this weekend," Biden told reporters at the White House.

Biden described what he called "stomach turning" reports of "parents butchered," "babies being killed," "entire families slain" and "women raped, assaulted and paraded as trophies."

At least 14 Americans died in the attack. National security adviser Jake Sullivan told reporters at a separate briefing that the US believed 20 Americans were missing but it remained unclear if they were being held hostage.

Biden said Hamas had threatened to execute hostages, including survivors of the Nazi Holocaust, in violation of "every code of human morality." He said the group's brutality and "blood thirstiness" was a reminder of the "worst rampages of ISIS."

"This is terrorism, but sadly for the Jewish people, it's not new," Biden said.

The incident had brought to the surface memories of antisemitism and the genocide of the Jewish people, he added.

In an apparent reference to Iran and Iranian-backed groups such as Lebanon's Hezbollah, Biden said he had a message for any country or organization thinking of taking advantage of the situation - "I have one word: don't."

Israel pounded the Gaza Strip on Tuesday with the fiercest air strikes in its 75-year-old conflict with the Palestinians, razing whole districts despite a threat from Hamas militants to execute a captive for each home hit without warning.

Biden promises Israel support

Biden spoke after his third phone call in four days with Israeli Prime Minister Benjamin Netanyahu.

The president said he told Netanyahu "our response would be swift, decisive and overwhelming" if the US had been attacked this way but that democracies also uphold the rule of law in wartime.

Terrorists "purposely target civilians and kill them," Biden said. "We uphold the ... law of war, it matters, there is a difference."

His remarks appeared to allude to the plight of ordinary Palestinians under Israeli bombardment in Gaza but he made no direct mention of them.

Biden outlined the US military assistance being sent to help Israel in its fight and said he would ask Congress to take urgent action. The White House is considering submitting a request to Congress that would include military aid to both Ukraine and Israel, said a person familiar with the matter.

Biden said American citizens were being held hostage by Hamas and that he had directed his team to share intelligence to help with hostage recovery efforts.

The White House on Monday said it expected to fulfill additional security requests from Israel as quickly as possible.

Biden promised replenishment of interceptors used by Israel's Iron Dome missile defense system as well as ammunition in addition to redoubling US military presence in the region.

The United States does not plan to put American military forces on the ground, White House national security spokesperson John Kirby told reporters earlier. Israel has called up an unprecedented 300,000 of its own reservists.

Biden and Netanyahu, despite differences on the way forward in the Middle East, have been thrust into a wartime partnership following the multipronged attack by Hamas militants from Gaza into Israel.

Biden also offered support for American Jews and said they should be allowed to worship in peace in the United States.

Biden said last month that antisemitism in the United States had risen to record levels.

Israel's embassy in Washington said the death toll from Hamas' weekend attacks had surpassed 1,000, dwarfing all modern Islamist attacks on the West since the Sept. 11, 2001, attacks on the United States. (Reuters-Yonhap)