The United States opened new peace talks on Wednesday that aim to stop Sudan’s catastrophic civil war, driven by a growing sense of urgency that the country’s deepening famine, which threatens millions of lives, could become the world’s worst in decades.

But Sudan’s military, one of the war’s two main belligerents, did not send a delegation to the negotiations in Switzerland, stymying hopes of a quick cease-fire in a destructive fight between the forces of two rival generals that has now lasted 16 months.

Famine was officially declared earlier this month in Sudan’s western Darfur region, and other areas are expected to follow. By one estimate as many as 2.5 million Sudanese could die from hunger by the end of September.

Appalled at the scale of the war-induced calamity in Africa’s third largest country, American officials say the peace drive is necessary, even if chances of a breakthrough seem slim.

Lt. Gen. Mohamed Hamdan, the paramilitary leader whose Rapid Support Forces are fighting the military, sent a delegation to the talks. But after a drone strike appeared to target the army leadership at a parade in eastern Sudan on July 31, the military chief, Gen. Abdel Fattah al-Burhan, spurned the meeting.

“We will not retreat, we will not surrender and we will not negotiate,” General al-Burhan told troops.

However, the United Arab Emirates, which has been accused of arming the R.S.F., did send a representative to the talks in Geneva, drawing criticism from some Sudanese commentators who said the Emirates should not have been invited.

The Emirates, which denies backing the R.S.F., has observer status at the talks, as does Egypt, long a political backer of Sudan’s military.

The Switzerland meeting picks up from earlier talks in Jeddah, Saudi Arabia, that failed to stop the fighting and ultimately stalled after the Sudanese army also boycotted them. Even before the talks started on Wednesday, however, American officials sought to temper expectations.

Tom Perriello, the U.S. envoy to Sudan, said in an interview before the talks began that his goal is to broker a cease-fire and to strike a deal for full humanitarian access across Sudan, where more than 10 million people have been forced from their homes and tens of thousands are estimated to have died.

If the military stays away, American officials hope to pressure its leaders to return to the table and also to draw global attention to a ballooning humanitarian crisis in which aid remains chronically underfunded.

“We need to start pivoting to a different set of solutions if we are to prevent a couple of million people from starving,” Mr. Perriello said.

The United Nations has received just one-third of the $2.7 billion it requested for Sudan, where people are dying because aid groups lack money, said Mohamed Refaat, country director with the U.N.’s International Organization for Migration.

“We see people who are dying, and who we have access to, who we can’t do anything for,” Mr. Refaat said in a video call with reporters from the de facto capital, Port Sudan, where all government functions have been moved since Khartoum was destroyed by fighting.

Previous peace efforts stalled after the Sudanese military insisted that the R.S.F. first drop its guns and abandon most of the territorial gains it has made since April 2023, a position the military has maintained in recent weeks.

“Military operations will not stop without the withdrawal of every last militiaman from the cities and villages they have plundered and colonized,” General al-Burhan said in a statement on Tuesday.

But American officials began to also see Saudi Arabia, their partner in peace talks, as part of the problem, according to two officials who spoke on the condition of anonymity to discuss sensitive diplomacy.

Saudi officials were doing little to encourage talks to resume in Jeddah, and seemed to relish the growing tension between the United States and Saudi Arabia’s own rival, the United Arab Emirates, over the Emiratis’ continued military support of the R.S.F., the two officials said.

In Geneva, Saudi officials are no longer leading the mediation and are instead listed as “co-hosts” alongside Switzerland.

American officials insist the talks will focus narrowly on a cease-fire and humanitarian access. Still, many in Sudanese civil society fear the talks could pave the way for a power-sharing deal that would hand Sudan back to the warring generals whose feud is destroying the country, instead of leading to the democratic transition many once hoped for.

The talks carry risk for the United States, too. Sudanese and even former American officials said the Biden administration’s flawed diplomacy in 2022 set the stage for the outbreak of war in 2023.

“Well-intentioned but counterproductive diplomatic efforts” by the United States and others “at best, failed to prevent the war and at worst contributed to its outbreak,” Payton Knopf, a former state department official who participated in some of those efforts, wrote this week.

Some Sudanese commentators have called the Geneva talks a “now or never” opportunity, noting that the chief organizer, Mr. Perriello, could be replaced, depending on the outcome of the presidential election in the United States.

Mr. Perriello still hoped he might persuade the Sudanese military to turn up. American mediators “are still waiting,” he wrote on X on Tuesday night. “The world is watching.”