JAKARTA, July 12 (Reuters) - Medical officials in Indonesia are still trying to pinpoint the source of infection after a 6-year-old boy died of bird flu at the weekend, a health official said on Thursday.

The boy, from the city of Cilegon in Banten province, suffered from high fever and breathing difficulties before he died of multi-organ failure on Sunday.

Contact with infected fowl is the most common way for humans to contract the H5N1 virus, but so far no clear link in this case has been established, officials said.

It is always a concern when the cause of a human infection cannot be traced as it makes infection control more difficult.

Runizar Ruesin, the head of the health ministry"e;s bird flu centre, said that at least 20 of chickens near the boy"e;s school had died suddenly.

"But we are still investigating whether he had a contact with sick or dead chickens in the neighbourhood," the official said.

A spokeswoman at the Jakarta hospital where the boy was treated said that along with some chickens dying near the boy"e;s school there were also water fowl in the school area.

"There are a lot of water fowl roaming near the boy"e;s school although they probably didn"e;t get into the school," said Tuty Hendrarwardati, a spokeswoman for the Sulianti Saroso hospital.

The boy, who had been suffering from fever before visiting relatives in the city of Bandung on June 25, had also been to the zoo, she said.

Researchers in Thailand detected H5N1 antibodies in 50 tigers at a zoo in 2004 -- proving that they were infected -- although they found no evidence of the virus in any of its zookeepers.

"He stayed in his relatives"e; house near Bandung zoo and visited it (the zoo) as well," she added.

The boy was later admitted to hospital in Tangerang, west of the capital, before being sent to Sulianti Saroso hospital on July 5, she added.

Bird flu is endemic in bird populations in most parts of Indonesia, where millions of backyard chickens live in close proximity with humans.

Experts fear if an easy means of transmission from human to human develops, there might be a pandemic affecting millions. Amin Subandrio, head of a panel of scientists at Indonesia"e;s bird flu commission, said on Wednesday the exact cause remained a mystery although so far there had been no indication of human to human transmission.

"A panel of experts will meet next week to discuss this. Normally we could trace it down to a contact with sick chicken, although it takes a while in some cases."

(Additional reporting by Adhityani Arga in Jakarta and Tan Ee Lyn in Hong Kong)