The Centers for Disease Control and Prevention increased its travel warning for “the biggest and most complex Ebola outbreak in history” as the White House claimed it’s doing everything possible to protect American citizens.

National Security Council senior director for development and democracy Gayle Smith told reporters on an administration call previewing next week’s Africa leaders’ summit in Washington that the outbreak is “obviously… a great concern.”

“We have been engaged with and in the region in working on this issue since March, when the first cases appeared. Obviously, there has been a decline in a couple of countries so that there’s greater attention on it. We are closely engaged with the leaders and the governments of the three countries most affected,” Smith said.

“We’re doing several things. One is ramping up our efforts to support a regional effort to deal with this outbreak and support, again, three governments who are doing a lot of things to contend with a real threat. Mind you, these are countries that have emerged — particularly Sierra Leone and Liberia — from years of war.  And so this is an uphill challenge for them,” she continued. “We’re also taking the necessary steps domestically to protect the American people. We have no plans to change the agenda of the summit, but we will obviously adapt as needed and in consultation with our partners, depending on their requirements.”

The CDC said it’s “rapidly increasing its ongoing efforts” in Guinea, Sierra Leone and Liberia.

“This is the biggest and most complex Ebola outbreak in history. Far too many lives have been lost already,” said CDC Director Tom Frieden in a statement. “It will take many months, and it won’t be easy, but Ebola can be stopped. We know what needs to be done. CDC is surging our response, sending 50 additional disease control experts to the region in the next 30 days.”

The precautions don’t include screening air passengers from affected countries.  ”It is important to note that Ebola is not contagious until symptoms appear, and that transmission is through direct contact of bodily fluids of an infected, symptomatic person or exposure to objects like needles that have been contaminated with infected secretions,” the CDC said.

Liberian President Ellen Johnson Sirleaf told CNN that the outbreak is “coming close to being a catastrophe.”

“We already have 300 persons in Liberia under surveillance, we have 47 deaths, 28 health workers dead, and I daresay it may be even greater in some of our other neighboring countries. This is very serious, it’s a disease that is unknown to us, it’s strange, it’s deadly, there’s no cure for it. Our people have at first — at — on the basis of no knowledge, resisted. The authorities deny today as people begin to die, we have — we are fearful, we are panicking, they are moving from one community to another,” said Sirleaf.

“We need everything. We need preventive gear, we need ambulances. More importantly, we need human assistance. We need technical assistance — doctors, nurses, experts that can train our own people as to how to deal with those who are sick, how we can encourage the preventive measures we are already taken — and we’ve taken a lot of them. We’ve quarantined places, we’ve closed schools, we’ve closed our borders, we’ve closed markets, we’ve done everything we can, but we’ve done is just not enough. What has happened in our sub region with our neighbors who have done a lot is still not enough. This is an international crisis and I hope that the international community will respond to it in that kind.”

White House press secretary Josh Earnest defended bringing at least one of two American aid workers stricken with Ebola back to the U.S. for treatment.

“These kinds of medevacs that are performed by private entities, by private organizations, yet facilitated by the U.S. government and government agencies, is consistent with what has been done in similar situations in the past,” Earnest said.

“In 2003, there were reports of American citizens overseas who contracted SARS. And the U.S. government facilitated the private transportation of those patients back to the U.S. so they could benefit from our modern medical infrastructure and have access to the kind of technology that could render lifesaving aid to them,” he added. “In 2007, there were Americans overseas who were at risk of contracting drug-resistant tuberculosis. Again, those individuals were transported through private means, but yet in a manner that was facilitated by the U.S. government, to return to the U.S. where they could get treatment.”