UNITED NATIONS — Bucking the threat of international sanctions, the president of South Sudan, Salva Kiir, refused to sign a peace deal to halt the nation’s civil war on Monday, the deadline for a final accord mediated by African leaders.

“If it is signed today and then tomorrow we go back to war, then what have we achieved?” Mr. Kiir was quoted as saying in a Twitter message posted by the South Sudanese government’s account.

South Sudan, which became the world’s newest nation four years ago to great fanfare, fell into a devastating civil war about 18 months ago. Thousands have died, more than 1.5 million people have fled their homes and nearly half the population is at risk of going hungry.

During his trip to Africa last month, President Obama convened a meeting of the region’s leaders to help press for an end to the fighting. They agreed to push the warring factions to sign a peace agreement by Monday and threatened both sides with sanctions or other measures if they did not comply.