The leaders of the United Nations and the World Health Organization expressed renewed alarm on Friday about Ebola’s tenacity in Africa and, in particular, its potential to ravage a fourth country, Mali, where they said hundreds of people had been exposed to an infected cleric who died last month.

At a webcast news conference from the World Bank offices in Washington, the United Nations’ secretary general, Ban Ki-moon, and the W.H.O.’s director general, Dr. Margaret Chan, also appeared to reset their schedules for containing the Ebola virus, which has sickened at least 15,351 people and killed 5,459, according to a W.H.O. update posted earlier Friday.

Mr. Ban said nothing about the goal of safely burying 70 percent of the dead and treating 70 percent of the sick by Dec. 1, and instead expressed hope that the outbreak could be contained by the middle of next year.

Mr. Ban’s special envoy on the Ebola crisis, David Nabarro, also expressed doubts about achieving the Dec. 1 treatment goal in comments at the United Nations. “Confident? No,” he told reporters outside the Security Council, which was holding a meeting on the Ebola crisis.