Israel got a warning from Egypt of potential violence three days before Hamas caught Israeli forces off-guard in a large-scale attack, the chairman of the powerful US House Foreign Affairs Committee said Wednesday.

"We know that Egypt has warned the Israelis three days prior that an event like this could happen," Republican Michael McCaul told reporters following a closed-door intelligence briefing for lawmakers on the crisis.

"I don't want to get too much into classified (details), but a warning was given," he said. "I think the question was at what level."

Israel is reeling from the deadliest attack in its 75-year history, as more than 1,500 militants stormed through the Gaza security barrier in their coordinated land, air and sea attack on the Jewish Sabbath.

Hamas' ability to remain undetected while preparing and launching such a big, complex assault from the closely monitored and heavily guarded Gaza strip represents an unprecedented intelligence failure for US ally Israel.

Israeli authorities say Hamas gunmen killed more than 1,200 people and wounded over 2,700 as they swept into small towns and kibbutzim and indiscriminately killed residents who hid in their homes or died defending their communities.

Israel has been relentlessly pounding Hamas targets in Gaza in response, and the war has already claimed the lives of more than 3,700 lives of Israeli and Palestinian civilians, soldiers and combatants.

In Washington, President Joe Biden has pledged to send more US munitions and military hardware and expressed revulsion at the "sheer evil" of the slaughter of civilians.

McCaul said the attack may have been planned as long as a year ago.

"We're not quite sure how we missed it. We're not quite sure how Israel missed it," he told reporters.

Cairo has not commented officially on suggestions that it may have offered an early warning.

But Egyptian media with close ties to the country's intelligence services on Wednesday quoted senior security sources denying Israeli press reports that such a warning was issued.