The White House confirmed that talks had started in Doha involving CIA Director William Burns and said negotiations would run through Friday.

"Today is a promising start," National Security Council spokesman John Kirby told reporters.

"There remains a lot of work to do. Given the complexity of the agreement, we do not anticipate coming out of these talks today with a deal," he said.

But he added: "Remaining obstacles can be overcome, and we must bring this process to a close."

"We need to see the hostages released, relief for Palestinian civilians in Gaza, security for Israel and lower tensions in the region, and we need to see those things as soon as possible."

President Joe Biden on May 31 put forward a ceasefire plan and last week joined the leaders of Qatar and Egypt in urging Hamas-Israel talks starting Thursday.

Hamas had initially made counterproposals but now says it wants the full implementation of Biden's plan, which includes an initial six-week ceasefire and the release of hostages.

Israeli Prime Minister Benjamin Netanyahu, whose far-right coalition partners oppose a deal, has reportedly put forward new demands that include Israeli control of the Gaza-Egypt border.

The White House would not discuss the issues under debate in Doha but insisted that they concerned implementation.

"We're at a point now where the framework is generally accepted, and where the gaps are in the execution of the deal," Kirby said.

Casting a cloud over the diplomacy have been tensions between Israel and Iran following the assassination in Tehran of Hamas's political leader Ismail Haniyeh, who was involved in ceasefire talks.

Iran has vowed to retaliate against Israel, prompting the United States to send more forces to the region. Kirby said it was unclear whether the pressure and ceasefire talks had led Iran to reconsider.

"I can't sit here and tell you for sure that there's been a decision to change their mind," he said.

"A few days ago, we had information, which we continue to have," he said, that "an attack could come with little or no warning, and certainly could come in coming days, and we have to be ready for it."