WASHINGTON — The Centers for Disease Control and Prevention placed their emergency operations center in Atlanta on the highest response level Wednesday because of the worsening Ebola outbreak in West Africa.The move comes as more cases of the deadly virus have migrated from rural villages and towns in Guinea, Liberia and Sierra Leone to urban areas in Nigeria, where it could spread more quickly.The number of Ebola deaths in the region has reached 932, according to the World Health Organization. The World Health Organization convened a special summit of international experts in Geneva on Wednesday to determine whether the outbreak constitutes a “public health emergency of international concern.” If so, WHO will recommend temporary measures to reduce the international spread of the disease.Next week, WHO will convene a panel of medical ethicists to explore using experimental treatments for Ebola patients in West Africa. No vaccine is available, but several are being developed.They include ZMapp, an experimental immune system booster made by Mapp Biopharmaceutical of San Diego. Two Americans who had been working with patients in West Africa and were stricken with Ebola have been hospitalized at Emory University Hospital in Atlanta and have reportedly been treated with ZMapp, even though the drug has never been tested in humans.President Barack Obama told reporters Wednesday that the U.S. will not send supplies of ZMapp to the epicenter of the outbreak in West Africa because it’s unclear whether the drug is actually helpful. It’s also “premature,” Obama said, to discuss fast-tracking the approval process for Ebola drugs until more information about their effectiveness is known.“We are in an unusual situation in this outbreak,” says Dr. Marie-Paule Kieny, WHO assistant director-general. “We have a disease with a high fatality rate without any proven treatment or vaccine. We need to ask the medical ethicists to give us guidance on what the responsible thing to do is.”Ideally, regulatory approval for new medications involves a series of small clinical trials in humans to determine whether the treatment is safe. The tests are later expanded to more people to determine their effectiveness and the best course of treatment.Obama said the outbreak continues to grow, in part because the affected countries have weak public health systems that have been overwhelmed by the challenge of quickly identifying and isolating Ebola patients. In addition, distrust of public health workers by local villagers have also hampered response efforts.“As a consequence, it spread more rapidly than has been typical with the periodic Ebola outbreaks that have occurred previously,” Obama said.While the U.S. must take the crisis seriously, Obama said, Ebola can be “controlled and contained very effectively if we use the right protocols.”As the U.S. and other international players continue to provide public health assistance in West Africa, Obama said it may become “entirely appropriate to see if additional drugs or medical treatments” can improve survival rates for the virus.“I will continue to seek information about what we’re learning about the drugs going forward,” he said.The CDC is sending 50 additional disease control experts to the four affected African nations to help treat patients and contain the disease. As of Monday, six CDC officials had been deployed to Guinea, 12 to Liberia, four to Nigeria and nine to Sierra Leone.CDC spokeswoman Karen Hunter said the movement of the center’s emergency operations center to Level 1 activation is “reserved for the largest-scale responses, which often require substantial agency-wide effort.” On Tuesday, the CDC issued an Alert Level 2 Travel Notice for Americans visiting Nigeria. A Warning Level 3 Travel Notice is also in effect for Guinea, Liberia and Sierra Leone. The CDC is advising Americans to avoid nonessential travel to these countries.