By Steve Holland

WASHINGTON (Reuters) - A long-sought Gaza ceasefire-for-hostages deal emerged at the end of an intense 96 hours of negotiations in Doha brokered by U.S., Egyptian and Qatari diplomats who persuaded Israel and Hamas finally to conclude the agreement.

A senior Biden administration official credited the presence of President-elect Donald Trump's incoming Middle East envoy, Steve Witkoff, as being critical to reaching the agreement announced on Wednesday after 15 months of war that devastated the Palestinian enclave and spread conflict across the Middle East.

Leading the U.S. side was President Joe Biden's Middle East envoy, Brett McGurk, who had been in the region since Jan. 5 working closely on what the official called a "very complex arrangement."

The agreement between Israel and Hamas got a big push across the finish line with Trump's repeated warnings there would be "hell to pay" in the Middle East if hostages held by the militant group were not released before his Jan. 20 inauguration, according to sources familiar with the matter.

The deal, after months of on-off negotiations, gained momentum after Israel and Lebanon's Hezbollah militia agreed to a ceasefire in November, and negotiations reached a boiling point over the last 96 hours, the administration official said.

A central obstacle was Hamas' refusal to acknowledge how many hostages it was holding or who among the hostages would be released in the first phase of the deal.

The hostages were among the group seized by Hamas during the militant group's assault on southern Israel on Oct. 7, 2023, when the gunmen also killed 1,200 people, according to Israeli tallies.

“That was the main issue just before Christmas time and we kept the pressure on Hamas and made clear there would not be a deal under any circumstance unless Hamas produced and agreed with the full list of hostages that would come out in the deal," the U.S. official said.

At the end of December, Hamas agreed to the list of hostages, which accelerated the final phase toward reaching a deal to free hostages in exchange for the release of some Palestinian prisoners held by Israel, the official said.

McGurk, who was in the region leading the U.S. team working to nail down the details, was later joined by Witkoff.

The main points included terms of the ceasefire, the sequencing of the release of hostages, the number of Palestinian prisoners Israel would release in exchange and future humanitarian aid to Gaza, the official said.

That phase of the negotiations became very intense.

Witkoff's involvement with McGurk in the final phase of the talks was a "fruitful partnership between the two of them to help nail down some of the final arrangements and bring them to a conclusion,” said the official.

Witkoff, a real estate investor close to Trump, also visited Israel to meet with Prime Minister Benjamin Netanyahu, and a person briefed on the deal said Witkoff "was able to pressure Netanyahu into accepting the deal and moving quickly."

The U.S. official said that until 3 a.m. local time on Wednesday, McGurk and Egyptian and Qatari mediators were meeting with the Israeli team on the second floor of the negotiations venue while Hamas representatives were downstairs.

The U.S. now hopes implementation can begin as soon as Sunday, the official said.

The deal outlines a six-week initial ceasefire phase and includes the gradual withdrawal of Israeli forces from Gaza and release of hostages held by Hamas in exchange for Palestinian detainees held by Israel, a separate official briefed on the negotiations told Reuters.

The agreement also requires 600 truckloads of humanitarian aid to be allowed into Gaza every day of the ceasefire, the official said.Israel's air and ground war in Gaza has killed more than 46,000 people, according to Gaza health ministry figures, with hundreds of thousands facing a severe humanitarian crisis.

(Reporting by Steve Holland; Additional reporting by Matt Spetalnick and Erin Banco; Editing by Don Durfee and Daniel Wallis)