The UN agency had earlier said it was "possible" that two sisters had caught the disease from their brother, sparking fears that the virus had mutated into a far more lethal form.

But in a follow-up investigation the WHO said virologists in Hong Kong had failed to find any human genes in the virus samples taken from the sisters.

This, it said in a statement on its website, indicated that the highly pathogenic H5N1 strain "has not changed to a form easily transmitted from one person to another."

The WHO has warned that H5N1 could kill millions across the globe if it combined with a human influenza virus to create a new highly contagious strain transmissible among humans.

The two sisters, who passed away on January 23 in a Hanoi hospital, are among 13 people to have died in Vietnam from bird flu.

They were part of a cluster of four cases of respiratory illness in the northern province of Thai Binh that included their brother and his wife.

The brother died shortly before his sisters were admitted to hospital and no samples were available from him for H5N1 testing. His wife was hospitalised on January 13 at the same time as the two sisters, but has since recovered.

In its latest update on the investigation into the cluster, the WHO said no illness has been reported in other family members, in the local community, or in health workers involved in care of these patients.

But it did not categorically rule out human-to-human transmission.