Joe Biden steps off Marine One upon return to the White House in Washington on Monday. AFP

Britain also said it would suspend some arms exports to Israel, citing a "clear risk" that they could be used in a serious breach of international humanitarian law.

In Washington, Biden met US negotiators working alongside Qatar and Egypt to try to secure a truce deal that would free hostages in Gaza in return for Palestinian prisoners held in Israel.

A White House statement said he expressed "his devastation and outrage" at the killings of the six hostages. Ahead of the meeting, Biden had said negotiators were "very close" to a final proposal to be presented to Israel and Hamas.

British Foreign Secretary David Lammy told parliament in London the UK would suspend 30 out of 350 arms export licences to Israel. The partial ban covered items "which could be used in the current conflict in Gaza," including fighter aircraft, helicopters and drones, he said.

Lammy also reiterated Britain's support for Israel's defence, stressing that the suspension would not have a "material impact on Israel's security."

'Believed in co-existence'

Alongside the surging domestic anger, diplomatic pressure has also grown, with Biden on Monday delivering some of his strongest criticism of Netanyahu. Asked by reporters if he thought Netanyahu was doing enough on the issue, he replied: "No."

At mass rallies late on Sunday, hostages' relatives called for a truce deal to help free the dozens still captive. Histadrut called a nationwide strike on Monday "for the return" of the remaining 97 hostages, including 33 the military says are dead.

Tel Aviv and the northern coastal city of Haifa heeded the strike call, but in Jerusalem and some other cities, life appeared to go on as usual.

AFPTV footage showed thousands of people turned out for a second consecutive night in Tel Aviv for protests demanding a ceasefire and hostage release deal.

Agence France-Presse