(Writes through with U.N., WHO interviews, Nigeria cases; changes dateline from Tokyo)

By Lesley Wroughton

WASHINGTON, Jan 12 (Reuters) - Bird flu has made a comeback in Nigeria and Japan and killed another person in Indonesia, perhaps revived by winter, experts said on Friday.

The H5N1 avian flu virus had continued to infect flocks in Indonesia and attacked the occasional person, but alarming headlines about its spread had died down in recent months.

"The transmission seems to be on the increase now," Dr. David Nabarro, bird flu coordinator for the United Nations, told Reuters in an interview.

"There is a pattern of seasonal increases in transmission in the months December through April over the past few years."

An official at a Jakarta hospital said a woman had died of bird flu and four people were being treated for symptoms for the H5N1 virus, which many scientists fear could mutate and trigger the next influenza pandemic in people.

Her death brought the mortality toll among people from the virus to 159, out of a total of 256 infected since 2003. Earlier this week, South Korea reported a worker had been infected last year without knowing it.

The dead woman's husband and son were among those hospitalized as suspected cases of infection, but there was no evidence of human-to-human transmission of the virus, a senior health official said.

In Nigeria, veterinary officials in white protective suits and masks culled more than 20,000 chickens at a farm in the far northwestern Nigerian state of Sokoto -- the first time the virus has been reported in birds there.

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.

"There was a lot of movement of people and poultry over the Christmas, New Year and Eid celebrations, and that is what has caused this new outbreak," said Junaidu Maina, head of Nigeria's livestock department.

BACK IN JAPAN

Japan reported its first suspected outbreak of H5N1 in three years. About 2,400 chickens died on a farm in the Miyazaki area of southwestern Japan in the past three days, an outbreak that initial tests showed was due to H5N1. If confirmed it would be the first in Japan since 2004.

Dr. Keiji Fukuda, coordinator for the global influenza program at the World Health Organization, said there was no evidence of any human-to-human transmission of the virus, including in Indonesia.

But the potential of the H5N1 virus to mutate and spread more easily between people remains a threat, although public complacency may have set in during several relatively quiet months, Fukuda said.

"It really looks like this has a kind of seasonal pattern that increases in winter months in the northern hemisphere," Fukuda told Reuters in an interview.

"I think that our concern about the virus and its potential ability to continue to evolve and become more transmissible to people remains. It's not that these new cases increase it; our concerns have been high all along," he added.

In recent weeks, China and Egypt have also reported new human cases to the U.N. agency.

In Vietnam, a government report said bird flu in poultry had moved closer to Ho Chi Minh City after an outbreak was confirmed in a fifth province.

Vietnam has had no human H5N1 cases since November 2005 but the virus re-emerged last month in Mekong delta poultry. Bird flu killed 42 of the 93 people infected in Vietnam from 2003 to 2005.

H5N1 has spread across much of Asia, into Europe, the Middle East and Africa. Migrating birds and poultry smuggling are believed to be some of the ways the virus has spread.

It spread very rapidly between January and March of 2006, leaving Asia for the rest of the world, but the spread had slowed in recent months -- as had news coverage.

"The fact that it is not part of the headlines quite so much is more a reflection of the familiarity of outbreaks of avian influenza rather than a reflection of the state of the disease, unfortunately," Nabarro said. (Additional reporting by Stephanie Nebehay in Geneva, Ho Binh Minh in Hanoi, Mina Valina Liem, Yoga Rusmana and Ahmad Pathoni in Jakarta, and Chikafumi Hodo and Teruaki Ueno in Tokyo)

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