"I think both our friends here in Washington and us think that this cannot be a game of extending all the time," said Smail Chergui, the African Union's commissioner for peace and security.
"This maybe is the last chance for them to respond -- first, to the will of their people. South Sudanese are tired of war," Chergui told reporters after talks between the AU and US in Washington, without specifying consequences if the efforts to form a government again fail.
South Sudan President Salva Kiir and rebel chief Riek Machar -- whose falling out in 2013, two years after independence, sparked a conflict that has since left hundreds of thousands dead -- had agreed to form a coalition government by November 12.
But African mediators led by Uganda agreed to give them 100 additional days, the second extension.
The United States -- which gives Juba about $1 billion per-year in mostly humanitarian aid -- issued a strongly worded statement saying it was reassessing its relationship with South Sudan.
Tibor Nagy, the top US diplomat for Africa, said the United States has "a lot of tools available and we will not hesitate to use them" but declined to say which actions Washington would take.
The two leaders, "and I would put quotes around the word 'leaders,' are obviously quite content with the current situation," Nagy told reporters.
"The international community is providing the food, the medicines, basically all of the human needs that are the responsibilities of governments to do. They're basically sitting back," he said.
While the United States has taken the most vocal line publicly, Nagy said he believed the world was onboard with pressing for action in South Sudan.
Nagy said he also discussed the political crisis in the mostly Christian nation Friday by telephone with Justin Welby, the Archbishop of Canterbury.