Sierra Leone will soon displace Liberia as the worst-hit of the West African countries ravaged by Ebola, the World Health Organization said Wednesday.

More than 600 new cases of Ebola were reported in the three countries most affected — Sierra Leone, Liberia and Guinea — in the week that ended Sunday, and more than half were in Sierra Leone, according to figures in an updated summary of cases and deaths on the W.H.O. website.

The W.H.O. update suggested that taken together, all three countries would miss the Dec. 1 target date for achieving important progress benchmarks — 70 percent isolation of patients and 70 percent of burials performed safely. Corpses of Ebola patients are extremely infectious and are an acute source of contagion.