Dr. Margaret Chan, the W.H.O. director general, was speaking as she met with the leaders of the three most affected countries — Guinea, Liberia and Sierra Leone — in Conakry, the Guinean capital, for the introduction of a $100 million plan to deploy hundreds more medical professionals in support of overstretched regional and international health workers.

“This meeting must mark a turning point in the outbreak response,” Dr. Chan said, according to a W.H.O. transcript of her remarks. “This outbreak is moving faster than our efforts to control it. If the situation continues to deteriorate, the consequences can be catastrophic in terms of lost lives but also severe socioeconomic disruption and a high risk of spread to other countries.”

She called the outbreak “by far the largest ever in the nearly four-decade history of this disease” and said it was “caused by the most lethal strain in the family of Ebola viruses.”