The Democratic Republic of Congo has confirmed the first case of Ebola in the eastern city of Goma, a major transport hub.
The World Health Organization (WHO) said the case could be a "game-changer" given the city's population of more than two million.
But the WHO expressed confidence in plans to deal with the diagnosis.
DR Congo's health ministry said a pastor tested positive after arriving in the city by bus on Sunday.
More than 1,600 people have died since the Ebola outbreak began in eastern DR Congo a year ago - the second biggest outbreak ever.
The WHO director-general, Tedros Adhanom Ghebreyesus, said an emergency meeting is being convened.
"We are confident in the measures we have put in place and hope that we will see no further transmission of Ebola in Goma. Nevertheless, we cannot be too careful," he said.
The health ministry said in a statement that there was a low risk of the disease spreading. It said everyone else on the bus - a driver and 18 other passengers - had been tracked down and they would be vaccinated on Monday.
"Because of the speed with which the patient has been identified and isolated, as well as the identification of all passengers from Butembo [where the bus came from], the risk of spreading to the rest of the city of Goma remains low," the statement said.
Some 3,000 health workers in the city have already been vaccinated.
The pastor travelled 200 km (125 miles) to Goma by bus from Butembo, where he had been with people with Ebola.