Hamas won't attend Gaza truce talks, hampering U.S. efforts to defuse escalating tensions across the region

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."

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.

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.

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.