(Adds EU official's comment)

DHAKA, March 23 (Reuters) - Bird flu has spread to six poultry farms near Bangladesh's capital, the government said on Friday, sparking a nationwide alert.

The United Nations also expressed concern.

C.S. Karim, the government adviser for agriculture and livestock, said among more than 42,400 poultry on the six farms in Savar, over 12,000 had died and another 21,000 had been culled over the past few days.

The H5N1 avian flu virus has not spread to other areas of the country and there was no cause for panic, he told a news conference. Savar is 25 km (14 miles) north of the capital.

Dr. Duangvadee Sungkhobol, representative of U.N.'s World Health Organisation in Dhaka, said: "We are very concerned because this is a highly densely populated country where people, animals and poultry live very close".

"The government has taken aggressive measures to stop the spread of the disease and that WHO has confidence it (the government) would be able to limit the spread," she told the same news conference.

The disease was confirmed through tests by laboratories in Bangladesh and Thailand, the government said late on Thursday.

Another U.N. official also expressed concern.

"Maybe the outbreak of avian flu started in the country weeks or months before but the authorities took a long time to confirm it."

"We are talking to the government and relevant agencies to find out the extent of the spread of H5N1 in Bangladesh," the official said on Friday. They asked not to be identified.

The European Union pledged assistance.

"The EU has kept funds ready for all of Asia in case they need to fight bird flu. All donors are contributing ... Bangladesh can use it to tackle the flu," Stefan Frowein, head of the EU delegation in Bangladesh, told a private television station on Friday.

"But such funds can be available through a single channel, the World Bank," he added.

Health experts had long expected an outbreak of H5N1 because the country is surrounded by India and Myanmar, which have reported bird flu infections.

Myanmar reported another outbreak of bird flu on Wednesday, saying a chicken farm had been hit outside the capital, where the H5N1 virus reappeared in four areas last month.

Bangladesh's dense population and large numbers of backyard poultry also increased the risks of outbreaks, experts have said.

The government has banned transport of poultry from affected areas, imposed constant monitoring of poultry farms across the country by joint forces led by the army and health checks on people working on the farms, Karim said.

"We have put the health network across the country on high alert and kept one specialised hospital ready to face any emergency," the government's health adviser, retired army major-general A.S.M. Matiur Rahman, said.

Syed Abu Siddiq, secretary of the Bangladesh Poultry Industries Association, said there were 125,000 small and large poultry firms in the country, producing 250 million broilers and 6 billion eggs annually.

Annual turnover was $750 million, he said.

About four million Bangladeshis were directly or indirectly associated with poultry farming. (Additional reporting by Nizam Ahmed and Masud Karim)

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