NEW YORK — U.S. health officials on Thursday warned Americans not to travel to the three West African countries hit by an outbreak of Ebola.

The travel advisory applies to nonessential travel to Guinea, Liberia and Sierra Leone, where the deadly disease has killed more than 700 people this year.

“The bottom line is Ebola is worsening in West Africa,” said Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, who announced the travel warning.