Washington DC - Secretary of State Antony Blinken on Tuesday urged Israel to make "fundamental changes" in its operations in the occupied West Bank after the military acknowledged its fire likely killed a US citizen activist there.

President Joe Biden later said he thought the killing of Aysenur Ezgi Eygi was an "accident," but Blinken called it "unprovoked and unjustified."

After an initially measured response to Eygi's death on Friday, pending a fact-finding exercise, Blinken said the US would raise it at senior levels with its key ally.

The investigation and eyewitness accounts make clear "that her killing was both unprovoked and unjustified," Blinken told reporters on a visit to London.

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"No one should be shot and killed for attending a protest," he said.

"In our judgment, Israeli security forces need to make some fundamental changes in the way that they operate in the West Bank, including changes to their rules of engagement.

"We have the second American citizen killed at the hands of Israeli security forces. It's not acceptable. It has to change."

Biden told reporters hours later, however, that the killing appeared to have been an accident.

"Apparently, it was an accident – it ricocheted off the ground, and she got hit by accident," Biden said without elaborating.

Eygi, who was 26 and also held Turkish citizenship, was killed as she attended the site of weekly demonstrations against Israeli settlements, which are illegal under international law but supported by right-wing members of Prime Minister Benjamin Netanyahu's government.

The Israeli military said it had found that it was "highly likely that she was hit indirectly and unintentionally by IDF (Israeli army) fire."

It added that the fire "was not aimed at her, but aimed at the key instigator of the riot."

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It said Eygi was killed "during a violent riot in which dozens of Palestinian suspects burned tires and hurled rocks towards security forces at the Beita Junction."

But Eygi's family rejected the military's version of events and called its preliminary inquiry "wholly inadequate."

"She was taking shelter in an olive grove when she was shot in the head and killed by a bullet from an Israeli soldier," they said in a statement.

"This cannot be misconstrued as anything other except a deliberate, targeted, and precise attack by the military against an unarmed civilian."