Washington DC - Israeli Prime Minister Benjamin Netanyahu was not doing enough to secure a deal for the release of Israeli hostages taken by Palestinian armed group Hamas, US President Joe Biden said on Monday.

Asked by reporters at the White House – where Biden was arriving for a meeting with US negotiators – if he thought the Israeli leader was doing enough on the issue, the president responded, "No."

Biden's meeting with the negotiators on the hostage-release deal comes after the deaths on Saturday of six captives in Gaza, including an American citizen.

"President Biden expressed his devastation and outrage at the murder, and reaffirmed the importance of holding Hamas's leaders accountable," a White House statement said after the negotiator meeting.

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His Vice President and the Democratic candidate for November's US presidential election Kamala Harris also condemned the killings, saying, "Hamas leaders will pay" but added it was "long past time for a ceasefire and hostage deal."

Biden and Harris – who is running to succeed him in November's US presidential election – were briefed by negotiators "on the status of the bridging proposal outlined by the US, Qatar, and Egypt," the statement said.

Ahead of the meeting, Biden had said negotiators were "very close" to a final proposal to be presented to Israel and Hamas.

Israeli Prime Minister Benjamin Netanyahu said on Monday that Hamas militants "executed" six hostages whose bodies were recovered from Gaza, shooting them "in the back of the head."

"These murderers executed six of our hostages, they shot them in the back of the head," he said during a press conference, rejecting the notion that Israel should respond with "concessions" in Gaza ceasefire talks.

Israel's health ministry said on Sunday that the six hostages were shot at close range shortly before their bodies were recovered from the Gaza Strip.

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As thousands of protesters gathered in Tel Aviv on Monday night for a second day of demonstrations criticizing the government, Netanyahu sought forgiveness for not being able to save them but insisted that leaving the corridor would not have led to a different outcome.

"I ask for your forgiveness for not bringing them back alive," he said.

"We were close but we didn't succeed. Hamas will pay a very heavy price for this."