HANOI (Reuters) - Vietnam has four more suspected human cases of the bird flu which has killed 49 people in Asia since the end of 2003, medical officials and a doctor have said.

Tests for the H5N1 virus were under way on a 25-year-old woman who was taken to Ho Chi Minh City´s Hospital for Tropical Diseases on Tuesday with coughing and fever after returning from Cambodia, a hospital official told Reuters.

He gave no further details of her trip to Cambodia, where officials said last week that bird flu killed a 28-year-old man, the country´s second victim of the poultry virus.

Two other Vietnamese suspects, a 41-year-old woman and a child, had fallen sick in the same district near the northern port city of Haiphong where five members of one family have been confirmed as suffering from bird flu.

"Their conditions are stabilising," a doctor in Haiphong´s Viet Tiep hospital said of the a couple and their three daughters.

The 41-year-old woman was a neighbour of the family and tests on her were under way, he said. Doctors said the five patients ate sick chicken when half of their 400 birds died earlier this month.

"A field investigation of this family cluster is under way," said a World Health Organization statement seen on Wednesday on its Web site www.who.int.

The U.N. health agency noted recent outbreaks of human cases in Vietnam included several clusters, mostly family members.

"There is currently no evidence that the H5N1 virus is spreading easily from person to person," it said, referring to fears the H5N1 virus could mutate into a form which could pass between humans easily and cause a global pandemic.

The fourth bird flu suspect, a 30-year-old man from the central province of Quang Tri, was at the Hue Central Hospital, deputy director Bui Duc Phu said.

Doctors were awaiting the results of tests on uim and a 16-month-old boy from Quang Binh province where a 5-year-old boy was confirmed to have bird flu and 37 other people had fevers, Phu said.