Diplomatic leaders are expected to gather Thursday for another round of intense negotiations to secure a ceasefire deal between Israel and Hamas, working under the threat of a wider war that would further devastate the Middle East. 

One key group won't be at the table: As of Wednesday, Hamas said it would not be taking part in the talks because it does not believe Israel has been negotiating in good faith.

"Going to new negotiations allows the occupation [Israel] to impose new conditions and employ the maze of negotiation to conduct more massacres," senior Hamas official Sami Abu Zuhri told Reuters. 

Israeli Prime Minister Benjamin Netanyahu says Hamas leader Yahya Sinwar has been the main obstacle to sealing a deal.

Two former ambassadors, a former special envoy and additional experts said the stakes could not be higher heading into Thursday's talks, agreeing a ceasefire is the best way to extinguish tensions threatening to boil over in the Middle East. They say the United States, with its diplomatic and military influence over Israel, has a considerable role to play.

"I don't think it's up to Hamas or even its allies at this moment in time," said Ali Vaez, director of the Iran Project at the International Crisis Group. "I think the one actor whose actions would be most consequential is the United States."

Tensions running high after killings

Ceasefire talks are set to resume in Doha, Qatar, on Thursday, with mediators from the host nation, Egypt and the United States. Hamas's absence does not mean progress won't be made, as it maintains open communication channels with Egypt, and Hamas has its chief negotiator based in Doha.

"Hamas is committed to the proposal presented to it on July 2, which is based on the UN Security Council resolution and the [U.S. President Joe] Biden speech, and the movement is prepared to immediately begin discussion over a mechanism to implement it," Abu Zuhri told Reuters.

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Ceasefire talks in Israel-Hamas war among most 'complex' Middle East challenges: former U.S. special envoy

David Satterfield, former U.S special envoy for Middle East humanitarian issues, says current negotiations are more complicated than any conflict he's seen in his 45 years on the file.

Tensions have been running higher in the Middle East after the killings of Hamas's political leader, Ismail Haniyeh, in Tehran and of top Hezbollah commander Fouad Shukur in Beirut in July. Israel claimed responsibility for the latter strike, but has neither confirmed nor denied its role in the blast that killed Haniyeh.

Iran has promised retaliation for Haniyeh's killing on its soil, while Hezbollah has vowed to seek revenge for Shukur's death. American and European leaders have called on Iran not to retaliate, warning such an attack could spark a larger regional war.

Prime Minister Justin Trudeau said Monday that there is a real risk of escalation and urged Canadians to leave Lebanon while it's possible because Ottawa may not be able to extricate everyone if the situation worsens.

Three senior Iranian officials told Reuters this week only a ceasefire deal in Gaza would hold Iran back from direct retaliation against Israel for Haniyeh's killing.

'Window for diplomatic action'

A senior adviser to U.S. President Joe Biden said Wednesday it's critical to take advantage of the "window for diplomatic action" on Thursday to end the war before escalations could spiral "out of control."

"The more time goes by of escalated tensions, the more time goes by of daily conflict, the more the odds and the chances go up for accidents, for mistakes, for inadvertent targets to be hit that could easily cause escalation that goes out of control," Amos Hochstein, tasked with shuttle diplomacy between Lebanon and Israel, told journalists on Wednesday.

Vaez said the U.S. is a critical player in negotiations because of his influence on Israel.

He said the nation has been vocal about its support for a ceasefire, but has so far been reluctant to use a major piece of leverage: its weapons supply to Israel. He said U.S. President Biden could use ammunitions as a negotiating chip, but has not done so since war broke out in October.

"The U.S. has tremendous leverage over Israel that it had so far been reluctant to deploy, and failure to do so again almost guarantees that there will be a descendant of chaos in a way that it would be much more difficult to contain and control," Vaez told CBC News in an interview.

David Satterfield, former U.S. special envoy for Middle East humanitarian issues and a former U.S. ambassador to Lebanon and Turkey, said the U.S. has "made it very clear that this war needs to stop" and that the ball is firmly in Hamas's court.

"The decision of whether or not there is a ceasefire — and there are still issues to be resolved here — lies with Yahya Sinwar, the leader of Hamas," said Satterfield.

"If there's an escalation, if there's an attack on Israel, that certainly can have a negative effect on ceasefire negotiations, no question about it…. But the linchpin, the hinge, is Sinwar's decision to negotiate in good faith." 

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U.S. to send guided missile submarine to Middle East amid mounting tensions

In an unusual move, U.S. officials announced they were deploying a guided missile submarine to the Middle East and accelerating the arrival of a carrier strike group led by USS Abraham Lincoln. The move comes as the region braces for possible attacks by Iran and its allies after the killing of senior members of Hamas and Hezbollah.

As tensions rise, the U.S. Navy has deployed warships and a submarine to the Middle East signal its willingness and ability to defend Israel in the event of wider war. 

Jon Allen, a senior fellow at the Munk School of Global Affairs and Public Policy and a former Canadian ambassador to Israel and Spain, said the U.S. is using the right two-pronged approach: working on a ceasefire and flexing its military muscle.

"I think they're doing what they should be doing," Allen said in an interview Tuesday. "I have no hesitation in saying they're doing everything possible [to get a deal] in the face of two difficult sides, the Israelis and Hamas. And so that's positive. 

"Then they're sending pretty clear messages that they are there and they're there to defend Israel on the one hand and to defend their own bases in the region and to try and make sure that there isn't a major regional conflict."

Vaez said a larger war would be unsurprising if a ceasefire deal can't be reached.

"It was predictable and predicted that if that war was prolonged, it would eventually expand and attack the size," he said. "And this is precisely what has happened."