ISLAMABAD, June 23 (Reuters) - An outbreak of the H5N1 bird flu virus has been found in chickens on a farm in Pakistan and about 25 people had been quarantined but no human cases had been discovered, government officials said on Monday.

The outbreak, at least the fifth in birds in Pakistan this year, was confirmed on Saturday at a poultry farm in Swabi town, about 60 km (40 miles) northwest of the capital Islamabad, the officials said.

"It was confirmed as H5N1 and thank God, no case of human infection has been reported," said a senior Agriculture Ministry official, who declined to be identified.

About 4,500 birds on the farm had been culled and the area disinfected, he said.

A Health Ministry official, who also declined to be identified, said about 25 people including farm workers and their close contacts were being monitored in quarantine.

Pakistan confirmed its first human death from the virus in a northwestern town in December.

Several outbreaks of the H5N1 strain of avian influenza in poultry and other birds have occurred in Pakistan, most in the country's northwest, since it was first detected in early 2006. (Reporting by Kamran Haider; Editing by Robert Birsel)

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