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MOSCOW (AP) - An outbreak of bird flu has been registered in southern Russia, officials said Monday, and news reports said the cases involved the H5N1 strain that scientists are watching for fear it could mutate into a more deadly form.

The virus killed birds at three farms or households in the Krasnodar territory, an agricultural region in the southern part of European Russia on the Black Sea, according to the federal agricultural oversight agency, Rosselkhoznadzor.

An agency spokeswoman said she could not give details about the particular strain involved, but the RIA-Novosti news agency cited the agency's chief spokesman, Alexei Alexeyenko, as saying it was H5N1.

Efforts were being made to prevent the spread of the disease, Alexeyenko was quoted as saying, without giving details.

Russia recorded its first cases of bird flu in Siberia in 2005, and outbreaks have since occurred further west. There have been no human cases of bird flu reported in Russia.

According to RIA-Novosti, bird flu cases were registered in 93 towns or settlements in Siberia and southern Russia last year. The report Monday was the first since August.

The H5N1 bird flu strain has killed at least 163 people worldwide since it began ravaging Asian poultry farms in late 2003, according to the World Health Organization. The virus remains hard for humans to catch, and most human victims were infected by direct contact with sick birds. But international experts fear H5N1 may mutate into a form that could spread easily among humans, and possibly spark a pandemic.