President Joe Biden pledged "rock solid" support for Israel on Saturday in the face of what he described as  "terrorist assaults" by Palestinian militant group Hamas, with an announcement on US military assistance expected imminently.

High-level discussions between US and Israeli officials on military aid were already underway, a US senior administration official said, with an announcement by Washington possible as early as Sunday.

Biden described the assault as "a terrible tragedy on a human level" and said he had spoken to Israeli Prime Minister Benjamin Netanyahu to underline his support.

"I told him the United States stands with the people of Israel in the face of these terrorist assaults," Biden said in a televised address from the White House.

"In my administration, support for Israel's security is rock solid and unwavering," he said.

"We'll make sure that they have the help their citizens need and they can continue to defend themselves."

Defense Secretary Lloyd Austin reaffirmed Washington's commitment, saying "over the coming days the Department of Defense will work to ensure that Israel has what it needs."

As the attacks threatened to trigger a wider conflict, Biden also warned "this is not a moment for any party hostile to Israel to exploit these attacks to seek advantage. The world is watching."

Biden stressed that Israel — which the United States has supplied with billions of dollars of arms — has "a right to defend itself" after Iran-backed Hamas launched air, sea, and land strikes.

The senior administration official, who spoke on condition of anonymity, stressed it was "too early to say" whether Iran was "directly involved" in Hamas' large-scale attack, but that there was "no doubt Hamas is funded, equipped and armed by Iran and others."

Meanwhile, top US diplomat Antony Blinken spoke with Palestinian president Mahmud Abbas — whose West Bank-based Fatah movement is a rival to Hamas — and "called on all leadership in the region to condemn" the attack on Israel.