The United States on Saturday condemned the attacks by "Hamas terrorists" against Israel and vowed to ensure the key US ally has the means to defend itself.

President Joe 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.

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

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 and its people" after the Iran-backed Palestinian militant group Hamas launched air, sea and land strikes.

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."

Trump weighs in

Former president Donald Trump weighed in Saturday, blaming Biden, without evidence, for indirectly funding the attacks.

"These Hamas attacks are a disgrace and Israel has every right to defend itself with overwhelming force," Trump said in a statement.

"Sadly, American taxpayer dollars helped fund these attacks, which many reports are saying came from the Biden Administration."

Trump's allegations reflected Republican claims that $6 billion released last month to Iran as part of a prisoner exchange deal was used to fund the Hamas attack.