By Steve Holland

WASHINGTON President Joe Biden on Tuesday will stress U.S. support for Israel as it reels from the killing of more than 1,000 people, including at least 11 Americans, from a surprise attack launched by Palestinian Hamas militants.

Biden is set to speak at 1 p.m. ET (1700 GMT) from the White House. He will express concern about the potential that some Americans are being held hostage by Hamas, an Iranian-backed Islamist group, a senior White House official said.

Israel pounded the Gaza Strip on Tuesday with the fiercest air strikes in its 75-year-old conflict with the Palestinians, razing whole districts to dust despite a threat from Hamas militants to execute a captive for each home hit.

Biden will speak after holding his third phone call in four days with Israeli Prime Minister Benjamin Netanyahu. He will outline in his remarks the U.S. military assistance being sent to help Israel in its fight, the official said.

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

The United States does not plan to put American military forces on the ground, White House national security spokesperson John Kirby told reporters. 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 multi-pronged attack by Hamas militants from Gaza into Israel.

Biden will also offer support for American Jews and say 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.

(Reporting by Steve Holland; Editing by Howard Goller)