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.