Three members of the same family are feared to have died from bird flu in Indonesia, according to Indonesian Health Minister Siti Fadillah Supari.
The deaths of a father and two young daughters in Tangerang on the outskirts of Jakarta, if confirmed, would be the first from avian influenza in the world's fourth most populous country.
Ms Supari said the first girl died several days ago and was already buried, and that tests on the other girl and the father had been conducted.
She said authorities were concerned about possible human-to-human transmission as there was no evidence of contact with poultry.
Authorities have taken blood samples of 315 people who had contact with the family. The mother of the two girls, aged one year and eight years old, was among those under observation, but so far she appeared healthy.
Last month, Indonesia reported its first human case in a poultry worker, but the man did not develop symptoms and is healthy.
Bird flu has killed 40 people in Vietnam - half of them since the H5N1 virus returned in December, state-run media said on Thursday.
Avian influenza, which arrived in Asia in late 2003, has also killed 12 Thais and four Cambodians. Millions of fowl have been slaughtered across the region in efforts to stamp out the virus.