The avian influenza virus that swept across East Asia early this year has reemerged in at least four countries in the region despite optimism among health and agriculture officials that the disease had been eradicated through the mass slaughter of chickens.

Since July, government officials have reported new cases among birds in Thailand, Vietnam and China. A fresh outbreak has also been confirmed in Malaysia, which had previously escaped the epidemic this year but is now the ninth Asian country to be afflicted by the highly lethal H5N1 strain.

"We think the H5N1 virus is entrenched in the rural environment in great swaths of Southeast Asia," said Peter Cordingly, a World Health Organization spokesman in Manila. "We are more concerned than at any time since the outbreak began in January that we have the makings of a crisis."

After several months with no report of human cases, Vietnam confirmed in August that two young children and a 25-year-old woman had died in separate outbreaks in the north and south of the country. Vietnamese officials had declared their country free of the disease in March despite the warnings of some U.N. experts that the assertion was premature.

Then, last week, officials in Thailand confirmed that an 18-year-old man who raised fighting cocks in a village north of Bangkok had died of bird flu. Four Thai children were hospitalized Sunday with symptoms of the virus.

"We were under the impression around the end of March or so that we had finally come to a situation where the avian influenza was over, but evidently it was not," said Samad Abdullah, WHO regional coordinator for communicable disease control. "We were a little over-optimistic."

The recent deaths raised the toll this year to 28, including 19 in Vietnam and nine in Thailand. A total of 39 people in those two countries have been diagnosed with the disease since the beginning of the year.

Though the number of new human infections remains modest, the virus continues to kill the majority of people it infects. Health officials are expressing worry that the longer the H5N1 strain remains active, the greater its chance to acquire genetic material from more common types of influenza, creating a virus that is highly lethal and readily passed among humans. That could occur if a person caught avian and human strains of flu simultaneously.

China raised an additional reason for worry late last month when it disclosed that scientists had detected two strains of bird flu, including H5N1, in pigs. Since pigs can contract human influenza, they could also serve as the source of a hybrid strain that could cause a pandemic.

When avian flu broke out this year, more than 100 million birds either died of infection or were killed in mass slaughters ordered by officials across East Asia. In some cases, governments called on soldiers to help carry out the killing.

But the virus managed to hang on, U.N. health and agriculture officials said, because much of the epidemic was centered in remote areas beyond the reach of a comprehensive culling effort. Officials said some poor peasants, threatened with the loss of their livelihoods, did not notify authorities when their chickens showed signs of infection.

Most of the recently reported cases are said by U.N. officials to be a continuation of the earlier outbreaks.

The infections among chickens, fighting cocks, quail and ducks in Malaysia are in five villages close to the border with Thailand, which had previously been afflicted. Officials said the disease was likely transmitted either by migratory birds or infected poultry smuggled across the border.

On Tuesday, Malaysian officials reported that the disease had spread to a village outside a quarantined zone established last month when the infection of local birds was confirmed.

Abdullah pointed to recent research showing that avian flu was endemic among such migratory birds as geese that are based in southern China and travel across Southeast Asia. Though these birds remain healthy, they could represent an ongoing source of infection for domestic poultry as they stop off at ponds and other watering holes common on chicken farms.

"One could not expect with one go to eliminate the virus completely," said Hans Wagner, a regional officer with the U.N. Food and Agriculture Organization in Bangkok. "We will have to battle this for quite some time."

He declined to speculate on how long it would take, but cited a top Vietnamese veterinary official who predicted it would require five years to eliminate the virus from his country.

U.N. officials said Asian governments are moving aggressively against the disease, in some cases faster than when it first appeared. Thailand, for instance, which initially sought to protect its major chicken exports by denying its poultry was infected, has killed at least 300,000 birds since July in a bid to contain the virus. Fourteen provinces are now being monitored.

Thai officials are debating this week whether to launch a campaign to vaccinate poultry against avian flu. While Wagner said this could be useful as one of several measures, thousands of Thai farmers have protested against the proposal, saying European consumers would object to eating chicken treated with chemicals. China and Indonesia are already carrying out widespread vaccinations, Wagner said.

A Filipino health worker inspected poultry confiscated in Manila in late August. Since July, new cases of bird flu have been found in Thailand, Vietnam, China and Malaysia.