By Colleen Jenkins

WINSTON-SALEM N.C. (Reuters) - A missionary doctor deemed to be at "some risk" for developing Ebola after returning from Liberia has been placed under a 21-day quarantine in North Carolina, his mission group and local health officials said on Monday.

Dr. John Fankhauser, 52, has shown no signs of the virus since arriving in Charlotte over the weekend but will be monitored for symptoms during the quarantine period ordered by the Mecklenburg County Health Department, Christian mission group SIM USA said.

This is the second quarantine the family physician from Ventura, California, will spend on SIM's campus in Charlotte, the group said.

In mid-August, he was isolated after returning to the United States from Liberia, where he treated Ebola patients, including two U.S. missionaries who became infected with the disease.

SIM said Fankhauser volunteered to go back to Liberia after another colleague, SIM missionary Dr. Rick Sacra, became ill with the virus. Fankhauser helped treat Sacra and then took over his duties at SIM's hospital outside of Monrovia after Sacra was evacuated to the United States for treatment, the group said.

A patient Fankhauser treated at that hospital while wearing protective gear later was diagnosed with Ebola, SIM said. The group did not say when he treated that patient.

According to the Centers for Disease Control and Prevention, a person who has had close contact with an infectious Ebola patient is considered at "some risk" for getting the disease.

Fankhauser's travel and contact with other people will be restricted during the quarantine, the health department said.

(Reporting by Colleen Jenkins; Editing by Susan Heavey)