Nigeria has said it has trained 800 volunteers to battle Ebola as fears rose that the worst-ever outbreak of the deadly disease could spread across Africa's most populous nation.

Authorities in the capital Lagos last week appealed for volunteers to make up for a shortage of medical personnel because of a six-week nationwide doctors' strike over pay.

Volunteers have so far been deployed to 57 districts of Lagos state but more are needed, particularly to treat those already infected with the disease, Hakeem Bello, a spokesman for Lagos State Governor Babatunde Fashola.

"We have trained some 800 volunteers in the area of contact tracing, sensitisation and treatment of the Ebola disease." said Bello.

Four people have died and six more are infected by Ebola in Nigeria as part of the worst-ever outbreak of the deadly virus, which has killed 1,145 people across west Africa this year.

Experts say Ebola is spreading out of control in the region, and the UN World Health Organization has declared the epidemic an international health emergency and appealed for global aid.

The disease erupted in the forested zone straddling the borders of Guinea, Sierra Leone and Liberia earlier this year and spread to Nigeria last month.

The districts of Kailahun and Kenema in eastern Sierra Leone have become the new epicentres of the outbreak, with charities and health authorities there scrambling to contain the spread of the disease.

US to provide extra aid

But officials fear an outbreak in the key regional hub of Nigeria could be far more dangerous, and US health authorities pledged this month to send extra personnel and resources to Africa's most populous country, as Nigerian doctors continue a strike across the country since July 1 to demand a pay rise and better working conditions.

Lagos' state government has stepped up a media campaign to raise awareness of how to prevent the spread of the disease, including radio and television advertisements and public health announcements.

Nigeria has also withdrawn its athletes from the Youth Olympics in the Chinese city of Nanjing as a result of the outbreak, state media reported as the games opened on Saturday.

The International Olympic Committee has barred athletes from Ebola-hit countries from competing in pool events and combat sports, affecting three athletes.

Canada and the United States are both sending consignments of largely untested drugs in the hope of saving hundreds of lives, but officials warn they are likely to have little impact.