The agriculture ministry said in a statement the 'orange' vigilance level was extended to the whole country after previously covering only the badly hit south-west.

The ministry said it had confirmed 20 outbreaks involving the H5 virus, of which at least six were the deadly H5N1 strain, since the first case was reported in Gimje, 260km south of Seoul, in early April.

It is investigating 14 more suspected cases, including one on a farm in Pyeongtaek, 70 km south of Seoul.

Officials have slaughtered 2.2 million chickens and ducks in and around infected farms, which are mainly in the South and North Jeolla provinces, a hub of the poultry industry.

The defence ministry said it was ready to help.

'As avian influenza is spreading, the military has decided to help slaughter and bury poultry in the infected areas,' a spokesman said.

'We are ready to deploy soldiers any time, even today, if requested by the local governments.' About 180 soldiers are already manning checkpoints in the Gimje area to help control movements in infected areas.

The National Emergency Management Agency ordered local firefighters to be ready to help with disinfecting vehicles and farms or other tasks, although it said they would not take part in culls.

'We'll do whatever we can do to prevent the bird flu outbreaks from spreading nationwide, which is now a national concern,' said Mr Kim Kook-Rae, a senior agency official.

Authorities have yet to fully explain why the outbreaks are not abating, but said on Tuesday that a poultry dealer was under investigation for breaching quarantine restrictions.

The dealer was found to have taken hundreds of ducks from an infected Gimje farm and supplied them to retailers and restaurants in other regions.

The agriculture ministry said it had located 141 restaurants or farms which had recently been visited by the dealer, and had so far culled poultry at 34 of the total. - AFP