The second American aid worker infected with Ebola in West Africa is due to arrive in Atlanta on Tuesday in serious medical condition, while a New York hospital completes tests on a man for the deadly virus.

Missionary Nancy Writebol, 59, is expected to arrive on Tuesday to be treated by infectious disease specialists in a special isolation ward at Emory University Hospital in Atlanta, according to Christian missionary group SIM USA.

Writebol took off from Liberia on Monday in a medical aircraft headed to Atlanta, according to a Reuters witness in that country.

The mother of two from Charlotte, North Carolina, is a longtime missionary who had been working for SIM USA as a hygienist tasked with decontaminating protective suits worn by healthcare workers inside an isolation unit at a Monrovia treatment center.

Her arrival was to come a day after Mount Sinai Hospital in Manhattan said it was testing a man who traveled to a West African nation where Ebola has been reported. He arrived at the emergency room in the morning on Monday with a high fever and a stomach ache, but was in "good condition", hospital officials said.

The New York City Health Department, after consulting with the hospital and the Centers for Disease Control and Prevention, said in a statement on Monday that "the patient is unlikely to have Ebola. Specimens are being tested for common causes of illness and to definitively exclude Ebola."

The patient added to concerns about the disease, which has killed nearly 900 people since February and has no proven cure. The death rate in the current epidemic is about 60 percent, experts say.

Emory's specialists have since Saturday been treating 33-year-old US doctor Kent Brantly, who also returned home after being stricken with Ebola during the emergency response to the worst Ebola outbreak on record.

Writebol and Brantly, believed to be the first Ebola patients ever treated in the US, served on a joint team in Monrovia run by Christian aid groups SIM USA and Samaritan's Purse. They are returning separately because the plane equipped to transport them can carry only one patient at a time.