Bird flu is on the march again across Asia as winter ends, but the battle against the killer virus is being hobbled by stark differences between the region's diverse countries, health experts warn.

While Vietnam and Thailand have been hailed as poster adverts in the fight against avian influenza, new cases in hotspot Indonesia and population giant China have highlighted concerns the disease will be hard to stamp out.

Many Asian nations have learned much since the H5N1 strain erupted here in 2003, spreading as far as Europe and Africa last year, but gaps remain and the threat of a pandemic remains real, epidemiologists say.

New human infections in China and Laos and recent animal cases in Myanmar, Pakistan, Afghanistan and Hong Kong are reminders that the virus "is still entrenched," said World Health Organisation (WHO) spokesman Peter Cordingley.

"If you take a broad look at the region, I would say it's pretty much the same as last year, when we saw bird flu move all the way to Africa.

"We're seeing signs of that already. We're seeing human cases in Egypt, and inside Asia the virus is obviously becoming quite active."

Bird flu is now popping up in new areas, and risk factors for transmission have increased, said He Changchui, the Asia-Pacific head of the Food and Agriculture Organisation (FAO) based in Bangkok.

"Last year it seemed we had more association with migrating wild birds, but this year we are also finding trade and the movement of poultry across borders played a very large role," he said.

"The virus doesn't know any borders, and due to globalisation and increased border trade and more regional integration in goods flows and people travelling, we can easily carry a virus from one country to another."