(CNN) -

The Ebola outbreak is much worse than official figures show and is "spiraling out of control," a leading U.S. official said Tuesday -- due in part, he said, to some countries that inadvertently have made it harder to corral the deadly disease.

Centers for Disease Control and Prevention Director Dr. Tom Frieden offered his stark commentary to CNN a day after Ellen Johnson Sirleaf, the president of Liberia, voiced dissatisfaction with the world response so far.

"In a way, we feel saddened by the response," President Sirleaf said.

More than 3,000 people have been infected by Ebola in Liberia, Guinea, Sierra Leone and Nigeria since the first documented cases in December, according to the World Health Organization. At least 1,552 have died.

Making it worse, Frieden said, is how other countries have turned their backs on those coming from countries where the outbreak is strongest, even if they don't realize it.

Measures to restrict flights and border crossings into the countries facing the outbreak were designed to contain the spread, but are having a paradoxical effect, he explained.

"This is making it really hard to get help in and to respond effectively to the outbreak," he said on CNN's "New Day."

"What we're seeing is a ... hugely fast increase in cases that's harder and harder to manage," he said. "The more we can get in there and tamp that down, the fewer cases we'll have in the weeks and months to come."

The president of Doctors Without Borders sharply criticized what she called the "global coalition of inaction" for focusing on insulating their nations instead of helping those in Africa who need it most. Dr. Joanne Liu said centers run by her group have turned away sick because they are too full, as an ever-increasing number of people develop symptoms of Ebola.

"States with the required capacity have a political and humanitarian responsibility to come forward and offer a desperately needed, concrete response to the disaster unfolding in front of the world's eyes ... rather than limit their response to the potential arrival of an infected patient in their countries," Liu said.

In her CNN interview, President Sirleaf of Liberia appealed for other nations to "please work as partners with us" by offering monetary, health care, logistical and other assistance, yes, but also by giving moral and personal support to a problem that's affected millions already.

"Give us hope by joining us in this fight," Sirleaf said. "Don't instill fear. We need that hope, we need that assistance, we need for Liberians to know that this war can be won."

Obama sends message to people of West Africa

The U.S. government tried to get front-and-center on the issue Tuesday, led by President Barack Obama.

Speaking "directly to ... the people of West Africa" in a video message, Obama said America's prayers are with those impacted by the disease and reiterated basic facts to educate locals on how Ebola spreads.

"Stopping this disease won't be easy, but we know how to do it," Obama said. "You are not alone. Together, we can treat those who are sick with respect and dignity, we can save lives, and our countries can work together to improve public health so this kind of outbreak doesn't happen again."

Obama insisted his government is working those in Africa to make a difference: To this point, White House spokesman Josh Earnest said more than 500 staff members from the CDC -- the U.S. health agency focused on infectious diseases, including Ebola -- are on the case, including 70 of the roughly 100 U.S. government personnel that are on the ground in hard-hit areas.

These government workers in Guinea, Liberia, Nigeria and Sierra Leone are doing things like "surveillance, contact tracing, database management and health education," Earnest said.

But they aren't the only Americans on the ground, putting themselves in harm's way to help others.

Many are there as part of humanitarian groups, like two who contracted Ebola earlier this summer. They were sent to Atlanta's Emory University Hospital, where they were treated and later released.

On Tuesday, a group called Serving in Mission announced that another American -- a doctor working in Liberia's capital --- had tested positive for Ebola.

The doctor, whose name was not released, was not treating Ebola patients and it's not known how he contracted the disease, SIM USA said. One of the other Americans infected with Ebola, Nancy Writebol, also was working on a SIM USA mission.

The unnamed American doctor immediately isolated himself upon the onset of symptoms, and is currently in an Ebola isolation unit, the group said.

"The doctor is doing well and is in good spirits," SIM USA said in a statement.

Sirleaf: 'We have no option but to succeed'