"We are starting to ramp up our staffing in Lagos," US Centers for Disease Control and Prevention spokesman Tom Skinner told AFP.
"We are really concerned about Lagos and the potential for spread there, given the fact that Lagos -- and Nigeria for that matter -- has never seen Ebola."
Nigeria became the fourth West African country involved in the largest Ebola outbreak in history when a dual US-Liberian citizen who was infected with Ebola traveled by plane to Lagos on July 20. He died five days later.
Eight people who came in contact with him have been diagnosed with Ebola, and two have died.
Ebola has killed 932 people in Sierra Leone, Guinea, Liberia and Nigeria since March, and has infected more than 1,700 according to the World Health Organization.
Experts say Ebola is out of control in West Africa, and the WHO on Friday declared the epidemic an international health emergency and appealed for global aid.
In Nigeria, President Goodluck Jonathan declared the control and containment of the Ebola virus "a national emergency," his office said in a statement.
He urged people to avoid large gatherings, and approved the release of $11.6 million to fund measures against the spread of the virus, including setting up isolation centers and increasing screening at borders.
- 'With a fury' -
Skinner said CDC personnel are in all affected countries, and that several US personnel already on the ground in Nigeria.
"We helped the folks in Lagos set up an emergency operations center similar to what we do here that can help with organizing the country's response to the outbreak," he said.
Earlier this week, the CDC issued an all-hands alert that allows the agency to direct more funding and staff to the crisis.
CDC chief Tom Frieden told lawmakers on Thursday that the agency already has 200 staff working on Ebola response, planned to "increase that number substantially."
At the same hearing before a House subcommittee, Ken Isaacs of the Christian aid group Samaritan's Purse warned that the situation in Nigeria was likely to worsen.
"Our epidemiologists believe that what we are going to see is a spike in the disease in Nigeria," said Isaacs, vice president of programs and government relations.
"It will go quiet for about three weeks and when it comes out, it will come out with a fury."
The incubation period of Ebola is 21 days, meaning it can take that long between initial exposure to the virus and the appearance of symptoms.
People become contagious as soon as they begin exhibiting symptoms, which include fever, muscle aches, vomiting, diarrhea, and sometimes bleeding.
Ebola first emerged in 1976, and there are no treatments or vaccines on the market.
- 'Stronger every day' -
A pair of American missionaries who fell ill with Ebola while treating patients in Liberia were given an experimental serum.
Their health has improved, though experts say it is unclear if the medication is the reason.
Kent Brantly, 33, and Nancy Writebol, 60, were flown out of Liberia and are now being treated at Emory University Hospital in Atlanta, Georgia.
"I am growing stronger every day, and I thank God for his mercy as I have wrestled with this terrible disease," Brantly wrote from his isolation unit Friday.
"I held the hands of countless individuals as this terrible disease took their lives away from them. I witnessed the horror firsthand, and I can still remember every face and name."