Nigeria has placed all of its ports and borders on red alert following the discovery of its first case of Ebola in the latest deadly outbreak to hit West Africa.

The Nigerian crackdown came as Sierra Leone authorities on Saturday searched for the first known resident of the capital Freetown with Ebola after her family removed her from a hospital when she tested positive for the deadly disease.

"We have had information that this patient was taken to a traditional herbalist for treatment. We followed the lead but we were still not able to get hold of the patient," a doctor at Freetown's King Harman Hospital told the Reuters news agency.

The outbreak that has killed more that 660 people across West Africa was found to have spread to the continent's most populous nation on Friday after a Liberian man with a high fever vomited aboard a plane to Nigeria and then died there, health officials said.

The 40-year-old man had recently lost his sister to Ebola in Liberia, they said.

It was not immediately clear how the man managed to board a flight, but he was moved into an isolation ward upon arrival in Nigeria on Tuesday and died on Friday.

Global health officials on Saturday said they were closely monitoring the spread of the disease and were working with governments and aid groups to try to stop it.

"Our thoughts and prayers are with those fighting the virus," Will Stevens, spokesman for the US State Department's Africa bureau, told news agency AFP.

"The US government continues to provide a comprehensive, multi-agency response to assist those countries affected by the Ebola virus outbreak," he added, saying multiple agencies were "contributing to the outbreak response efforts".

Passengers monitored

Onyebuchi Chukwu, the Nigerian health minister, said Friday's was the first case of Ebola to be confirmed in Nigeria since this year's outbreak emerged.

The disease already has hit Guinea, Liberia and Sierra Leone.

"All ports of entry into Nigeria, including airports, seaports and land borders, are placed on red alert," he said.

"Ministry of Health specialists have been positioned in all entry points. Active surveillance has also been stepped up."

Authorities are investigating anyone who may have come into contact with the deceased, Chukwu said.

"We have already gotten in touch with all the passengers,'' he said. "We are monitoring and investigating."

Bernice Dahn, the Liberian chief medical officer, identified the Ebola victim as a government official with the finance ministry.

Ebola has now infected at least 1,093 people in Sierra Leone, Liberia, Guinea, and now Nigeria, according to the World Health Organisation.

Source: Agencies