(CDC/AP Photo)

As two American relief volunteers infected with the Ebola virus make " slow improvement" at an Atlanta hospital, the director of the Centers for Disease Control and Prevention told Congress that even with a full recovery it will be impossible to know whether an experimental drug has had any impact on their health.

Lawmakers convened a rare summer hearing today to examine the international community's response to the outbreak across West Africa, as well as deliberate whether ZMapp, an experimental serum mixture of three antibodies, could help stem the deadly outbreak if it was more widely available.

"The plain fact is that we don't know whether that treatment is helpful, harmful or doesn't have any impact," Dr. Tom Frieden testified. "We're unlikely to know from the experience of two or a handful of patients whether it works."

WATCH: Ebola Outbreak Puts CDC on Highest Alert

The suffering Americans, Nancy Writebol, a nurse who contracted Ebola while volunteering in Liberia, as well as her fellow missionary, Dr. Kent Brantly, arrived in the U.S. over the past several days under quarantine.

Frieden added that the U.S. government is looking into ways to expedite development and production of promising drugs, but he added that he didn't want to create "any false hopes" that a cure or vaccine is on the horizon.

"If there's a new treatment, we will do everything we can to help get it out to those who need it most," Frieden said. "But right now, we are months or at least a year away."

Ken Isaacs, a vice president at Samaritan's Purse, one of two primary organizations working to combat the virus on the front lines in Guinea, Sierra Leone and Liberia, slammed the international response to the outbreak and predicted a spike in the coming weeks of cases in Nigeria, where the disease is now believed to have spread.

"It is clear to say that the disease is uncontained and it is out of control in West Africa. The international response to the disease has been a failure," Isaacs said. "Our epidemiologists believe that what we are going to see is a spike in the disease in Nigeria and that it will go quiet for about three weeks and when it comes out, it will come out with a fury."

Although the CDC is expanding its ground operation in the region, Isaac warned that the U.S. must provide additional resources necessary to potentially accommodate newly deployed American health care workers as they arrive in the zone.

"If we are going to expect people, including the CDC people, to go abroad and put their life on the line, there has to be some assurance that we are able to care for them if they are sick," Isaacs said. "That may be a regional health facility that is exclusive to those citizens and those workers, or that may be a demonstrated capacity to get them home. But one airplane with one chamber to get them back is a bit of a slow process."

Isaacs added that the full impact of the outbreak "has not been realized" and cautioned the disease could potentially present a national security risk to the United States.

"If we do not fight and contain this disease in West Africa, we will be fighting disease and containing it in multiple other countries around the world," he said. "The truth is the cat is most likely already out of the bag."

Rep. Chris Smith, R-New Jersey, chairman of the House Foreign Affairs Subcommittee on Africa, Global Health, Global Human Rights and International Organizations, convened the uncommon hearing during the congressional recess to examine efforts to combat the Ebola outbreak, which is suspected or confirmed in more than 1,700 people, causing more than 900 fatalities in West Africa.

WATCH: Ebola Numbers on the Rise

"Many people are not cooperating with efforts to contain disease," Smith said. "To those who say we have no plan, I would say that planning is definitely underway and it's being done so very aggressively. Still, there is much more that needs to be done."

While Frieden called the current outbreak "a crisis" that is "unprecedented" in its scope, he expressed optimism the epidemic can be contained.

"We can stop Ebola. We know how to do it," Frieden assured lawmakers. "It will be a long and hard fight and the situation in Lagos, Nigeria, is particularly concerning, but we can stop Ebola."