Indonesia's bird flu toll jumped to 30 on Wednesday after the World Health Organisation (WHO) confirmed five more people had died of the virus in the world's fourth most populous nation.

The WHO said four of the confirmed deaths came from a cluster in North Sumatra: a 19 and 17-year-old male, a 29-year-old female and an 18-month old baby. A fifth person, a 25-year-old male, was infected but alive, the agency said.

"WHO is carefully investigating these cases, as any possible cluster case raises increased suspicions that human-to-human transmission may have occurred," spokeswoman Sari Setiogi told AFP.

"However the current investigation that we have has no evidence of further spread beyond the cluster, so that's quite good news for us because it tells that the virus is not spreading further," she said, warning however that the investigation was still ongoing.

She said two more people were possibly involved in the cluster: a 37-year-old woman from whom samples were not taken was considered the first person to have shown symptoms while results on a 10-year-old male who had also died were still being awaited.

The fifth death reported Wednesday was from Surabaya in East Java, she said.

I Nyoman Kandun, director for the health ministry's communicable disease control centre, told AFP earlier that the five deaths had come from a related family.

"They were apparently infected at a family event," he said. Kandun said earlier this week that the five had died within days of each other over the past three weeks.

A doctor at the Adam Malik hospital in Medan said the bird flu patient, Jones Ginting, was recovering, the state-run Antara news agency reported.

Doctor Alwinsyan Abidin told Antara that Ginting was no longer suffering a high fever or breathing difficulties and was eating and walking.

The vast majority of deaths in Indonesia have so far occurred in the capital Jakarta and its surroundings, where many people live close to poultry despite the urban environment.

Indonesia has witnessed more bird flu deaths than any other country this year. It has the world's second highest number of fatalities since 2003, after Vietnam.

Bird flu has now killed at least 120 people since late 2003, mostly in Asia. Experts fear the virus may mutate into a form that can pass easily between humans, sparking a pandemic.

An expert from the UN Food and Agriculture Organisation said this week that public ignorance about the dangers of bird flu as well as poor co-ordination between various levels of government were the biggest obstacles Indonesia faces in its fight against the virus.

While other Southeast Asian nations like Thailand and Vietnam have had considerable success controlling the spread of the virus, Indonesia has one of the region's highest rates of infections among poultry, Laurence Gleeson said.

He said its epidemic was threatening to spread to Pacific nations after poultry in the easternmost province of Papua was recently found to be infected.

The WHO's Asia chief Shigeru Omi this month urged Indonesia to work harder at grassroots level to combat the deadly H5N1 strain of bird flu.