Samantha Power said before arriving in Guinea 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."
Latest WHO figures
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.
In Conakry on Sunday, the US envoy met with religious leaders and Ebola survivors at Guinea's largest mosque and assured them of US support.
"We're in this with you for the long haul," she said.
More than 10 000 people have contracted the virus in west Africa, according to the latest World Health Organisation figures.
Another country in the region, Mali, was scrambling to prevent a wider outbreak after a two-year-old girl died from her Ebola infection following a 1 000km bus ride from Guinea. She was Mali's first recorded case of the disease.
'Feel like a criminal'
In the United States, An American nurse who was placed in quarantine after caring for Ebola sufferers in Sierra Leone has complained she was made to feel "like a criminal" upon arrival in New Jersey.
Kaci Hickox, who later tested negative, was the first person to be placed under a mandatory 21-day quarantine for medical staff returning to parts of the US who may have had contact with Ebola patients in west Africa.
The new rules took effect in the states of New York, New Jersey and Illinois on Friday, the same day Hickox returned.
"This is not a situation I would wish on anyone, and I am scared for those who will follow me," Hickox wrote in The Dallas Morning News.
"I am scared about how healthcare workers will be treated at airports when they declare that they have been fighting Ebola in west Africa. I am scared that, like me, they will arrive and see a frenzy of disorganisation, fear and, most frightening, quarantine."
She said she was being kept outside the main hospital building, with only a hospital bed, a non-flush chemical toilet, and no shower.
"To put me in prison is just inhumane," she told CNN Sunday.
New York Mayor Bill de Blasio Bill de Blasio attempted to quell the firestorm over Hickox's outspoken remarks, saying "this hero was treated with disrespect, was treated with a sense that she had done something wrong, when she hadn't."
Ambassador Power expressed concern that the new quarantine policies were "haphazard and not well thought out".
US health officials also voiced concern that the new rules could discourage much-needed medical personnel from volunteering to go to west Africa.
"When they [health workers] come back, they need to be treated in a way that doesn't disincentivise them from going there," National Institute of Allergy and Infectious Diseases director Anthony Fauci told CNN.
President Barack Obama told Americans on Saturday they must be "guided by the facts, not fear" after a 33-year-old US doctor returned from Africa became the first Ebola case in New York City.
Meanwhile, Australian authorities said early on Monday that a teenager who was in isolation in hospital had tested negative for Ebola after she developed a fever following her arrival from Guinea.
The 18-year-old, who arrived in Australia 12 days ago with eight other family members, had been in home quarantine in Brisbane before she developed a raised temperature.
Mali sees no symptoms
In Mali, the WHO warned of an "emergency" situation after a girl died from Ebola following a bus ride from Guinea to Mali with her grandmother.
But an advisor to the Malian health ministry told AFP the 43 people placed under medical observation in Kayes in western Mali - where the girl died on Friday - "do not show for the moment any signs of the illness", said doctor Lamine Diarra.
About a dozen other people are also being observed in the capital Bamako, where the little girl had spent about three hours visiting with relatives on the way to Kayes, he added.
Mauritania meanwhile reinforced controls on its border with Mali, which effectively led to the frontier being closed, according to local sources.
Ebola is spread though close contact with the sweat, vomit, blood or other bodily fluids of an infected person.