Bird flu has spread to ducks and chickens near Vietnam's border with China, the Agriculture Ministry said on Friday.

The outbreak in mountainous Cao Bang province took to 16 provinces and two cities the number of areas struck by the H5N1 bird flu virus since it re-emerged in Vietnam in early May.

The ministry said tests confirmed the presence of H5N1 in a poultry farm near China's Guangxi province, which shares a 311 km (195 miles) border with Vietnam.

The virus killed 87 ducks and chickens at a farm in Thong Nong district on June 11. All poultry on the farm were then destroyed.

On Tuesday, officials said two women, aged 28 and 29, had been infected by the H5N1 virus in the northern provinces of Thanh Hoa and Ha Nam. Doctors said the 28-year-old from Ha Nam province was in serious condition and needed a respirator to breathe.

Four people have been found to be infected since May, the first human cases of H5N1 in Vietnam in a year and a half.

The two women and an infected slaughterhouse worker were being treated in a Hanoi hospital.

A 30-year-old man whose infection was reported on May 24 was discharged from the same hospital on June 4. Last week, doctors said he had been cured.

Bird flu has killed 42 people out of 97 known infections in Vietnam since the virus re-emerged in Asia in late 2003. The World Health Organisation (WHO) has not confirmed the latest four infections.

Globally, the H5N1 virus has killed 191 people out of 312 known cases, according to a tally by the WHO. Hundreds of millions of birds have died or been slaughtered.