The deadly H5N1 strain of bird flu has appeared in remote Taraba state in eastern Nigeria, but in most other parts of Africa´s most populous country the spread of the virus is slow, officials said on Friday.

The first African country to be hit by bird flu, Nigeria has not reported any human cases of the disease although experts warn surveillance may not be completely effective and cases may have gone undetected.

The detection of the virus in Taraba means bird flu is now present in 14 of Nigeria´s 36 states and in the Federal Capital Territory. The state that had most recently joined the list was Lagos in April.

"We have identified it in three places in Taraba, one commercial farm and two places where there are backyard poultry," said Lami Lombin, director of the National Veterinary Research Institute which tests for bird flu.

"There are a few places in Plateau and Kaduna where we are still getting new outbreaks. Elsewhere all the samples we´re getting are negative," Lombin told Reuters by telephone.

Plateau and Kaduna, which form a strip of north-central Nigeria, were among the first four states where H5N1 was detected in February. Taraba borders Plateau to the east.

In the first few weeks after the first cases were found, bird flu spread rapidly in poultry across the country despite measures to contain it such as culling and quarantine.

Veterinary officials say it would not be safe to consider the disease contained, but recent evidence suggests measures to prevent its spread are working better than at the start.