A senior official on US President Donald Trump's Board of Peace has criticised Israel's continuing air strikes on Gaza, as well as the actions of the Palestinian armed group Hamas.
Nickolay Mladenov warned that a collapse of last October's ceasefire would be "a point of no return" for the region.
He was speaking to the UN Security Council after Israel rejected the 15-point roadmap for Hamas's disarmament, which was announced by the Board of Peace last month.
Hamas has agreed to give up its weapons in exchange for a phased Israeli troop withdrawal. But Israel has insisted that the group must fully disarm before any pullout.
At least 1,303 Palestinians, including 300 children, have been killed in Israeli attacks in Gaza since the US-brokered ceasefire began, according to the territory's Hamas-run health ministry, whose figures are considered reliable by the UN.
The Israeli military says four of its soldiers have been killed in Palestinian attacks in Gaza over the same period.
The Board of Peace's high representative for Gaza told the Security Council on Wednesday that he and Israeli Prime Minister Benjamin Netanyahu had agreed at their last meeting on a mechanism through which his government's remaining questions would be settled and that committees had been established for that purpose.
Mladenov said that process had not stalled but warned that risks remained, including the continued killing of Palestinian civilians in Israeli strikes.
"A ceasefire in fact under which civilians are still buried is not yet one that the people of Gaza can really feel. Every use of force that does not answer an immediate threat, and every incident that the deconfliction mechanism was not asked to resolve, costs this process something very hard to buy back," he added.
Mladenov acknowledged that Hamas too had yet to meet fully its commitment to cease all military activities.
"Every weapon carried, every tunnel still worked and every convoy still obstructed sets the process back and hands an argument to those who want to resume war," he said.
He repeated that under the new roadmap, Israeli troop withdrawal from Gaza "follows verified decommissioning" of Hamas weapons handed over to the Palestinian technocratic committee that will govern Gaza under the Board of Peace's supervision.
"It is sequenced and reciprocal," he said.
That is a key difference from comments made by President Trump's envoy, Jared Kushner, after meeting Israel's prime minister two weeks ago.
He said Israeli withdrawal would be discussed only once the demilitarisation of Gaza was "done" - appearing to change the plan in line with Israel's demands.
Israel's representative told the Security Council meeting that it was committed to last October's US-brokered plan for ending the Gaza war, but she insisted it could not succeed while Hamas still had weapons, tunnels and other military infrastructure.
"Disarmament must come first," she stated.
The Palestinian representative said Israel was killing Palestinian civilians with impunity in Gaza and called on the international community to protect them without delay.
Mladenov warned that if the ceasefire was allowed to collapse, "there will be no roadmap to return to, there will be no administration to deploy, and there will be nothing left of the ground to rebuild".
"That would not be a setback. It would be a point of no return for everyone."
The war was triggered by the Hamas-led attack on southern Israel on 7 October 2023, in which about 1,200 people were killed and 251 others were taken hostage.
Israel responded by launching a military campaign in Gaza, during which more than 73,430 people have been killed, according to the territory's health ministry.
