ABUJA, Nigeria — In an ominous warning as fatalities mounted in West Africa from the worst known outbreak of the Ebola virus, the head of the World Health Organization said on Friday that the disease was moving faster than efforts to curb it, with potentially catastrophic consequences, including a “high risk” that it will spread.

The assessment was among the most dire since the outbreak was identified in March. The outbreak has been blamed for the deaths of 729 people, according to W.H.O. figures, and has left over 1,300 people with confirmed or suspected infections.

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. “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.”