Samantha Power said before arriving in Guinea on Sunday that too many leaders were praising the efforts of countries like the United States and Britain to accelerate aid to the worst-affected nations, while doing little themselves.
"The international response to Ebola needs to be taken to a wholly different scale than it is right now," Power told NBC News.
She said many countries "are signing on to resolutions and praising the good work that the United States and the United Kingdom and others are doing, but they themselves haven't taken the responsibility yet to send docs, to send beds, to send the reasonable amount of money."
Besides Guinea, Power will travel to Sierra Leone and Liberia -- the three nations that account for the vast majority of the 4,922 deaths from the Ebola epidemic.
More than 10,000 people have contracted the virus in west Africa, according to the latest World Health Organization figures.