Experts are warning that the risk of a human bird flu pandemic remains just as real and is in fact probably growing.

They say as the bird flu virus becomes further entrenched in poultry in more countries, the risk of a pandemic remains and is probably expanding.

At a meeting of 150 experts organised by the World Health Organization (WHO), in order to update its guidance to countries on how to boost their defences against a deadly global epidemic, it has been revealed that the H5N1 avian flu virus has now infected flocks in much of Asia, Africa and parts of Europe.

Health experts are concerned that the virus will mutate into a form that passes easily from person to person, triggering a pandemic which could kill millions.

Dr. Supamit Chunsuttiwat, a disease control expert at Thailand's health ministry says the risk of a pandemic remains and the highly pathogenic H5N1 virus persists on three continents and had also caused human cases in Indonesia, Egypt and China this year.

Dr. Supamit says there are concerns that the spread through migratory birds has not stopped and once the virus is established in birds, it is difficult to eliminate the risk to humans if countries do not have good control of transmission in birds.

According to the WHO, even though the avian flu virus rarely infects people, there have nevertheless been 382 human cases worldwide since 2003, and 241 have been fatal.

The coordinator of WHO's global influenza program, Keiji Fukuda says the threat of pandemic influenza has not diminished and the timing remains a matter of speculation.

Although most countries have drawn up pandemic plans, the level of preparedness varies which is a concern, as a pandemic could overwhelm healthcare systems, particularly in poor countries.

The WHO says the risk of international spread through asymptomatic air travellers is a very real concern and the organisation acknowledges that it will need to improve its guidance to give people and countries better tools to deal with such an event.

The WHO aims to revise its 2005 guidance to its 193 member states by the end of the year and has set up two global stockpiles of the antiviral Tamiflu, containing 5 million treatment courses donated by the Swiss drugmaker Roche, for use in the event of a pandemic.

The WHO is also developing a vaccine stockpile which, when a vaccine has been produced should initially contain 150 million doses.

As many as 16 companies are currently working on vaccines to prevent bird flu infection in people and some are said to be close to regulatory approval; these newer generation vaccines need less antigen and contain adjuvant but can still stimulate an immune response.

Experts say until a pandemic virus has emerged large-scale commercial production of a vaccine will not start because the vaccine must closely match the strain.

The WHO currently uses six phases of pandemic alert to gauge levels of threat and the world is currently in phase 3, meaning that a new influenza virus sub-type is causing disease in humans but is not yet spreading efficiently.

The H5N1 strain of bird flu virus currently circulating first appeared in Asian poultry in late 2003 and to date almost all human cases have been linked to contact with infected birds.

Indonesia has been the worse hit with 108 deaths so far and is seen by many experts as a potential source for a pandemic.

The WHO says health regulations which came into effect in 2007 oblige countries to report new disease threats with global public health significance, such as new flu subtypes; they also allow the WHO to act on credible information sources, rather than just being reliant on official government channels.

The health regulations countries are designed to curb the spread of a pandemic and include assuring access to medical centers, control of airports and other points of entry and preparations to isolate sick people and quarantine contacts.