The outbreak was reported on Monday and killed 590 ducks on a farm in Ha Nam province, the animal health department said in an online report.

Tests on dead chickens floating in a river in a nearby commune also tested positive for the disease, the report said.

Outbreaks were once again reported on Vietnam's small farms in October, raising fears of a new nationwide epidemic as winter approaches.

Farms have been hit in central Quang Tri province, northern Cao Bang, Nam Dinh and Ha Nam and southern Tra Vinh provinces.

The United States said on Wednesday it will provide an additional US$10.5 million to Vietnam for avian and pandemic influenza prevention and control, the American embassy in Hanoi said.

Since 2005, Washington has granted more than US$22 million to Vietnam for prevention and containment efforts, it added.

Vietnam, one of the nation's worst hit by the virus, had managed to control the situation with mass culls and vaccination campaigns.

But the deadly virus again strongly hit the country in May this year before slowing and then reappearing on farms in October.

Bird flu has killed 46 people in Vietnam since it first hit in 2003, while the global death toll is 205, according to the World Health Organization.