New strains of the H5N1 virus caused some of the fresh outbreaks of bird flu in Thailand and Laos and they appear to have spread from southern China, the Food and Agriculture Organization said on Thursday.

Vigorous control measures must be implemented to prevent further spread of the disease in birds and poultry, the U.N. agency said in a statement.

Recent outbreaks of avian flu in northeastern Thailand and neighboring Laos were caused by a H5N1 virus strain previously not detected in the region, but similar to a strain found in southern China, it said.

"Poultry trade across borders is continuing in Southeast and East Asia despite well-known risks," the FAO said.

A bird flu outbreak last month in Thailand´s central province of Pichit was caused by the same strain circulating in the area for 2 to 3 years. A 17-year-old youth died in the outbreak.

"The H5N1 virus thus remained alive in central Thailand in a reservoir of birds and poultry, most probably a mixture of backyard chickens, ducks and fighting cocks," Laurence Gleeson, manager of the FAO´s bird flu center in Bangkok, said in the statement.

The virus has spread to Europe, the Middle East and Africa and is known to have killed 139 people, according to the World Health Organization. Indonesian officials also reported the death of a nine-year-old girl from the disease in West Java this week.

Clusters of human cases have fanned fears that the virus could mutate into a form that passes easily between people and cause a pandemic in which millions could die. So far, there is no evidence such a mutation has occurred.