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The administration of President Donald Trump blamed Hamas on Sunday for the renewed attacks on Gaza, saying the group had rejected an "acceptable deal".
Special envoy to the Middle East Steve Witkoff said the US-led "bridging proposal" would have extended the ceasefire while negotiations continued.
"Hamas had every opportunity to demilitarise, to accept the bridging proposal that would have given us a 40 or 50 day cease fire, where we could have discussed demilitarisation and a final truce," Mr Witkoff told Fox news.
"There were all kinds of opportunities to do that, and they elected not to, and this becomes the alternative, and it is unfortunate.
"So this is on Hamas, the United States stands with the state of Israel," he said.
The comments come after Israel resumed air strikes on Gaza after two months of relative calm in the war that had killed more than 50,000 Palestinians in nearly 18 months.
Israel's army launched strikes by air and on the ground, effectively abandoning the ceasefire that was meant to seethe remaining hostages being held by Hamas released in exchange for Palestinian prisoners, and negotiate a permanent end to to the conflict.
Hamas has accused Israel of breaking the ceasefire agreement, which took effect in January, by refusing to enter into the second phase of the agreement, which includes the withdrawal of Israeli troops from Gaza.
The war was sparked on October 7, when Hamas launched an attack on Israel killing 1,200 people.