A leading bird flu expert urged Indonesia on Thursday to do more animal surveillance to curb the H5N1 virus, which has killed 46 people so far in the country, the highest death toll anywhere in the world.

The virus was first detected in the Indonesian archipelago in late 2003 and is now endemic in almost all of its provinces, due in part to reluctance to carry out mass culling of birds.

"Indonesia should do more animal surveillance and understand how the virus behaves. It is a place that causes concern," said Guan Yi, a microbiology professor at the University of Hong Kong.

"Since 2003, it has had no new introduction of the H5N1. It has only one strain, but its problem is like China´s. It can´t clean it up," said Guan, who has studied the virus since 1997, when it made its first known jump to humans in Hong Kong.

Guan´s call came as 4 people were admitted to an Indonesian hospital with bird flu symptoms in an area of West Java that has seen a series of confirmed and suspected cases in humans.

At least two people in Cikelet, about 90 km (55 miles) south of the provincial capital of West Java, Bandung, have been confirmed to have died from the H5N1 virus in recent weeks.

Indonesia has so far recorded 60 bird flu cases, 46 of them fatal. More troubling, there have been clusters of human infections and in at least one case, health experts have been able to confirm direct human-to-human transmission of H5N1.

Although the virus remains largely a disease in birds, there are fears it might mutate and become easily transmissible among humans, setting off a pandemic that could kill millions.

While Guan thought Indonesia was in as bad a situation as China when it came to controlling the disease in birds, what set them apart was the strength of their governments.

"Even China is better than Indonesia in some ways. China´s government is at least strong, it can just lock people up. But Indonesia can´t do that, it is so dispersed. It has no control," he said, referring to cases where Indonesians suffering from bird flu had simply refused to be treated and left the hospital.

In contrast, Guan said Thailand´s method of getting villagers to conduct animal surveillance was effective and he recommended the practice be adopted by other nations. Thailand has seen a re-emergence of H5N1, which killed two people in recent weeks.

But he expressed concern over nations where access to information is not readily available.

"There are countries that are black boxes; Laos, Myanmar, Cambodia. They are forgotten by the world. These are dangerous places," he said.

A senior World Health Organization (WHO) bird flu official told Reuters in Geneva on Thursday there was no evidence so far that the two groups of suspect cases in Indonesia might point to human-to-human transmission of the virus.

"It looks like these are exposures to infected birds," said Keiji Fukuda, coordinator of WHO´s global influenza programme, on the sidelines of a health forum in the Swiss city.